Handle vertex_uvs if they are present in default prepass fragment shader

[IceSentry]

Objective

• Enabling AlphaMode::Opaque in the shader_prepass example crashes. The issue seems to be that enabling opaque also generates vertex_uvs

Fixes #8273

Solution

• Use the vertex_uvs in the shader if they are present

[prime31]

Minor nitpick: the comment “Opaque cube using the StandardMaterial” should say “CustomMaterial”

[nicopap]

Could using a common Vertex definition between the prepass and the mesh shader help avoid similar issues in the future? I wrote a patch at https://gist.github.com/nicopap/be825768467ad2285ad5a345aa502786

The branch is at https://github.com/nicopap/bevy/tree/cleanup-prepass

[IceSentry]

Could using a common Vertex definition between the prepass and the mesh shader help

Maybe, but they technically aren't the same. For example, vertex_color shouldn't really be part of the prepass. There's also things in the prepass vertex output that aren't part of the main pass vertex output like the previous_world_position used for motion vectors