[Flutter GPU] Uniform and texture binds marshal the slot name across the FFI boundary on every call #189819

RenderPass.bindUniform and RenderPass.bindTexture pass the uniform's name as a Dart string handle on every call. The native side then allocates a std::string via tonic::StdStringFromDart and resolves the binding with a string-keyed map lookup, per bind, per draw, per frame, even though the resolution result never changes for a given shader between reloads.

In draw-heavy scenes this is a measurable share of main-thread frame time. In a renderer submitting ~10k draws per frame (2 uniform binds and 1 texture bind per draw), the name marshal and lookups account for roughly 7-12% of the CPU render phase on macOS/Metal in profile mode.

The reflection data is immutable between shader reloads, so the binding can be resolved to a stable index once and reused. Proposal in the linked PR.