Mesh renderer type does not work on macOS

[peterdijkstra]

Mesh renderer type paints magenta, suggesting a shader issue. Not sure what, my shader knowledge is superficial at best

Tested on both the old builtin rendering pipeline and the new URP

[mjkkirschner]

the mesh shader seems to break for some reason on macOS metal - in the unpack_color function - I tried alternative bit shifting techniques to unpack the data but still could not get the correct colors.

the generated metal code looks pretty strange - for example there is a multiplication by a constant after the unpack.

I don't understand why this does not effect the procedural version though as it seems to use the same shaders on the builtin pipeline.

[meckzqz]

Using an int instead of a uint for unpack_color's parameter will fix this issue on macOS.