The tesselator code uses some defines that it expects to be in the GL
headers such as GLAPI and GLAPIENTRY. These are used to mark the entry
points as exportable on each platform. We don't really want the
tesselator code to use these but we also don't want to modify the C
files so instead they are #defined to be empty in the stub glu.h. That
header is only included internally when building the tesselator/ files
so it shouldn't affect the rest of Cogl.
GLES also doesn't have a GLdouble type so we just #define this to be a
regular double.
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