GLAPI is the desktop OpenGL equivalent of OpenGL ES's GL_APICALL macro.
They need to be consistent to prevent CLang from producing errors when
compiling with -Werror,-Winconsistent-dllimport.

Change-Id: Iceb73f42f52628a31aba7ce18fe60221529f890c
Reviewed-on: https://swiftshader-review.googlesource.com/19108Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>