Add corner case for the use of WebGPU canvases for `TexImageSource`

For WebGPU canvases the size of the drawing buffer for canvases isn't the size of the canvas itself, but whatever was passed to `GPUCanvasContext.configure`. WebGL methods importing `TexImageSource` should be updated to describe this.

The issue points to WebGL method descriptions that import TexImageSource; search the specification text for those methods and the WebGPU canvas drawing-buffer wording. Update the affected descriptions to cover the size supplied to GPUCanvasContext.configure, then verify that each method explains this corner case.

Written by the indexing model from the issue text.