[Flutter GPU] Compute shaders on Metal and Vulkan - #193805

… sets

Shader.getStorageBufferSlot looks up a compute shader's storage buffer by
name, caching its reflection index per hot reload like UniformSlot.
Binding sets take storage buffers. A bound view is checked in Dart: it
covers the block, holds whole elements of a trailing runtime-sized array,
and starts at a multiple of the new
GpuContext.minimumStorageBufferByteAlignment. HostBuffer aligns each
emplacement to both the uniform and the storage alignment.

Fixes flutter#188479.

GpuContext.createComputePipeline builds a pipeline from one compute
shader, with the shader's workgroup size and descriptor set layouts. A
workgroup size over the device limits throws with the reason. A pipeline
whose shader hot reloads rebuilds at its next dispatch. GpuContext
exposes the compute limits: workgroup invocations, workgroup size,
workgroup count and shared memory size.

CommandBuffer.createComputePass returns a ComputePass that follows the
pass lifecycle. It binds pipelines, storage buffers, uniforms, sampled
textures and binding sets, which persist across dispatches, and
dispatch takes workgroup counts. A dispatch throws if a binding the
shader declares is unbound. A compute shader keeps the resources
Metal's compiler removes, so it takes the same bindings on every
backend; the pass skips the backend bind for them on Metal.

GpuContext.supportsCompute is true on Metal and Vulkan and false on
OpenGL ES, where compute pipeline and pass creation throw. A Metal
compute pass now ends its encoder when it is released unended, like
render and blit passes.

Fixes flutter#188480.

Adds compute shaders to the test shader bundles and gpu_test.dart
coverage for a compute pass writing vertices that a render pass draws,
dependent dispatches without a barrier, binding sets, sampled textures,
storage buffer bind validation, unused resources that must be bound on
every backend, the pass lifecycle, the compute limits, the workgroup
size limit, and the supportsCompute gate on OpenGL ES. A reload test
checks that an existing ComputePipeline runs reloaded shader code.

Part of flutter#188481.

The prefix sum test shader now uses a literal workgroup size, and the
dispatch contract and synchronization tests add six shaders.

Instead of widening the blit barriers and the render pass outgoing
dependency to include the compute stage, the first dispatch of a Vulkan
compute pass records one barrier that waits on earlier graphics, transfer
and host writes, so only command buffers that use compute pay for it.
The blit pass and render pass builder are back to their state on master.
Adds tests for compute passes that read a texture or a buffer written by
a blit pass.

ComputePipelineDescriptor::ValidateWorkgroupSize returns an absl::Status
and builds its message only on error. In Flutter GPU, ComputePass,
ComputePipeline, Shader::RegisterSync and CommandBuffer::EndComputePass
return absl::Status or absl::StatusOr, with messages built by
absl::StrCat. The FFI entry points convert the status to the error
string or bool Dart already receives. Binding maps use
absl::flat_hash_map, and a null pipeline in ComputePassVK::SetPipeline
or a null pass in EndComputePass is a DCHECK.

Adds docstrings to the storage buffer reflection types, the shader
bundle schema, ComputePassVK (in doxygen style) and the other types the
change adds, and documents the lifetime of the binding returned by
Shader::GetStorageBufferAt. Groups the compute limits in Capabilities
into a ComputeLimits struct, pops the debug label with a
ScopedCleanupClosure, adds argument comments to the Vulkan calls,
removes the performance-move-const-arg suppressions and simplifies the
binding set lookup in the Flutter GPU compute pass, and builds the test
shader bundle descriptions in BUILD.gn from one entry per line.

Shader.getStorageBufferSlot throws an ArgumentError for a name the shader
does not declare, so StorageBufferSlot.sizeInBytes and
runtimeArrayStrideInBytes are no longer nullable. If a hot reload removes
the storage buffer, using the slot throws a StateError. The reload test
bundle now replaces ComputeScaleWithUnused with a shader that drops its
Unused storage buffer, to test that.

Adds a test that draws a computed triangle covering part of the viewport
and checks pixels inside and outside each of its edges, rather than only
that the corners are one color. Names the lime and red pixel values the
compute tests read back, and notes that lime is the full-intensity green.

…buffer

The storage buffer slot and the Vulkan bound resource type now have
default values, for cppcoreguidelines-pro-type-member-init. The blit
readback test writes its device-private texture through a staging buffer,
since Metal disallows CPU writes to private textures.

Master replaced fml::Status with absl (flutter#194161).