UI materials have some usability friction when used to bevy shaders

[NiseVoid]

UI materials currently aren't as low friction to use as they probably should.

The issues I've found so far are:

• Materials properties end up in bind group 3. In the regular shader pipeline in bevy these are in bind group 1.
• The UVs in VertexOutput are vec3, so they cannot be passed directly to functions that sample textures uvs.
• The way the custom shader example sets up the material produces a new asset every render, the example should show a better approach