This fixes inconsistencies shown in the tests I added for the
WM_GETDLGCODE.  The tests covered different cases handled by the
current implementation in order to show that the native implementation
is simpler for all these cases.

This makes more sense than the other way around.

This allows us to skip BGRA->RGBA color conversion for vertex attributes if
this extension is present.

This should also be a little bit faster.

Remove some uncessary code and use more descriptive parameter names. Note that
max_vertex_idx isn't the same as "minIndex + calculatedNumberOfindices - 1"
but rather "minIndex + numberOfVertices - 1". calculatedNumberOfindices just
specifies the number of vertices that's drawn, while numberOfVertices
specifies the range of vertices that's drawn.