[Flutter GPU] Pass lifecycle: one recording pass per command buffer, ended by the next command #193867

Part of #188474.

A Flutter GPU pass never ends until submit(). lib/gpu creates the backend pass when the Dart pass is created and encodes every pass at submit, so on Metal two passes in one command buffer have two command encoders open at once, which Metal does not allow. Apps work around this today with one command buffer per render pass, and compute passes (#188480) would hit the same limit.

This implements the pass lifecycle decided on #188474. It changes render passes as well as compute, so it lands on its own.

Changes

• A command buffer records one pass at a time, and passes execute in the order they are created.
• Creating the next pass, or recording a copy or readback on the command buffer, ends the current pass. submit() ends the last one.
• When a pass ends, its Metal or Vulkan encoder ends at that point, so a command buffer can hold any number of passes. OpenGL ES keeps encoding at submit, in the same order.
• RenderPass.end() ends a pass explicitly and is harmless to call twice.
• Any call on an ended pass throws a StateError naming what ended it, for example "RenderPass was ended when createRenderPass() was called on the same CommandBuffer".

Testing

gpu_test.dart on Metal, Vulkan and OpenGL ES:

• Two render passes in one command buffer both render correctly.
• Using a pass after the next pass was created, after a copy, or after end() throws.
• Calling end() twice is harmless.

Dependencies

None.