Previously, we linked against libOSMesa through the linker and
then used dlopen to link against libGL. This is not how libOSMesa
is intended to be used. Instead, we use dlopen on libOSMesa
(via bluegl), which then will map the correct GL methods for us,
and for the OSMesa functions, we also map those functions
from libOSMesa (instead of relying on compile-time linker).
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