renderdiff: enable webgpu and improvements (#9219)

- Allow for presets to override selected models when presented
   in order.
 - Add 'gltf' for model search path
 - Add a few simple gltf models to the 'base' preset
 - Improve UI so that missing tests do not generate any html
   bits.
 - Add documentation on using the viewer
 - Add renderdiff documentation to the project webpage.

RDIFF_BRANCH=pf/renderdiff-enable-webgpu

test/renderdiff/src/render.py

@@ -22,7 +22,6 @@ import concurrent.futures
 from utils import execute, ArgParseImpl, mkdir_p, mv_f, important_print
 
 import test_config
-from golden_manager import GoldenManager
 from image_diff import same_image
 from results import RESULT_OK, RESULT_FAILED