Reference tests (reftests) for WebGPU canvas presentation.These render some contents to a canvas using WebGPU, and WPT compares the rendering result withthe "reference" versions (in `ref/`) which render with 2D canvas.This tests things like:- The canvas has the correct orientation.- The canvas renders with the correct transfer function.- The canvas blends and interpolates in the correct color encoding.TODO(#918): Test all possible color spaces (once we have more than 1)TODO(#921): Why is there sometimes a difference of 1 (e.g. 3f vs 40) in canvas_size_different_with_back_buffer_size?And why does chromium's image_diff show diffs on other pixels that don't seem to have diffs?TODO(#1093): Test rgba16float values which are out of gamut of the canvas but under SDR luminance.TODO(#1093): Test rgba16float values which are above SDR luminance.TODO(#1116): Test canvas scaling.TODO: Test transferControlToOffscreen, used from {the same,another} thread