Avoid nested structs in the types used to pass data from vertex to fragment shader functions

[maxbrunsfeld]

This seems to be required in order for the shaders to compile successfully on macOS versions <= 10.15 (Catalina). In these versions, when nested structs are used, we get errors like this:

for vertex function return types:

  metal shader compilation failed:
  ./src/platform/mac/shaders/shaders.metal:40:8: error: invalid return type 'QuadFragmentInput' for vertex function
  vertex QuadFragmentInput quad_vertex(
         ^
  ./src/platform/mac/shaders/shaders.metal:37:5: note: field of illegal type 'GPUIQuad' declared here
      GPUIQuad quad;
      ^

for fragment function inputs:

  ./src/platform/mac/shaders/shaders.metal:59:5: error: invalid type 'QuadFragmentInput' of input declaration with attribute 'stage_in' in a fragment function
      QuadFragmentInput input [[stage_in]]
      ^
  ./src/platform/mac/shaders/shaders.metal:37:5: note: field of illegal type 'GPUIQuad' declared here
      GPUIQuad quad;
      ^

[as-cii]

Pretty weird error but excellent work figuring this out! ⚡