[Impeller] Add feature to allow attribute mapping of uint32s host-side to vec4s shader-side

Once the GLES 2 renderer lands, impellerc could be extended to map the 4
color bytes to vec4s in the shader, i.e.:
glVertexAttribPointer(2, 4, GL_UNSIGNED_BYTE, false, 4, nullptr);
...as opposed to:
glVertexAttribPointer(2, 4, GL_FLOAT, false, 16, nullptr);

impellerc could e.g. detect some kind of special naming convention hint
from the attribute name to support this case:
in vec4 vertex_color__byte;