Allow for usage of GLTF primitives that have no texcoords

[4-rodrigo-salazar]

I purchased a model from cgtrader.com where the gltf file describes the model using just base colors (no textures and no texcoords on primitives, which is valid gltf), and it panics when bevy tries to load the gltf and compile the pipeline.

Bevy GLTF loader creates the mesh with no UV attributes, as expected. When it attempts to compile the pipeline we get an error saying that it expected a Vertex_Uv attribute but it's not supplied by the mesh.

thread 'Compute Task Pool (5)' panicked at 'Attribute Vertex_Uv is required by shader, but not supplied by mesh. Either remove the attribute from the shader or supply the attribute (Vertex_Uv) to the mesh. ', crates\bevy_render\src\pipeline\pipeline_compiler.rs:223:17

This changes makes it so that the forward shader uv attributes get enabled with STANDARDMATERIAL_ALBEDO_TEXTURE since the gltf loader uses PbrBundle/StandardMaterial. The uv attribute today is only used when STANDARDMATERIAL_ALBEDO_TEXTURE is enabled in the fragment shader.

Tested FlightHelmet and my gltf which has no texcoords, both work now.

[cart]

Hmm I think the problem shouldn't be resolved in shaders. That puts to much burden on the shader author to support every potential input mesh.

Instead I think this should either be fixed by:

1. inserting empty uvs into the mesh asset before rendering (a manual user action)
2. automatically inserting zero-ed out buffers for meshes with missing vertex attributes. Mesh overhaul with custom vertex attributes #592 #599 used to do this, but it was removed and tabled for a later date. This feels like the most user-friendly approach, but it will take some finesse to get the UX and performance right.

Post processing your mesh with (1) in app code should be a decent workaround. (2) is the solution I want to shoot for.

[4-rodrigo-salazar]

Cool, i'll try to take a look at implementing what you described as 2.

Thanks for thinking about this!

[cart]

Closing this as it will require a different implementation (as discussed above)