Implements MultiplanarExternalTextureTransform to allow transforming a
texture_external binding into two texture_2d<f32> bindings and a uniform
buffer binding. Transforms textureSampleLevel and textureLoad calls with
a texture_external parameter into custom functions that can handle both
single-plane RGBA or bi-planar YUV. Includes tests.
Bug: dawn:1082
Change-Id: Icb6d8b0f3773feca01c833171f07230c3531f3aa
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/68620
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>