createGraphics does not work in WebGPU mode

[awu7]

Upon inspection of the code, the WebGPU mode image function checks for a textureIndex property, which is not present on the OffscreenCanvas returned by createGraphics.

[quinton-ashley]

I'm thinking that webgpu mode will not be able to adequately support creating or drawing graphics that are dynamically drawn to, at least not in an efficient way, since copyExternalImageToTexture is too slow. I'll investigate this later.