doxygen: Plumb through gallium/ to automated documentation
doxygen: Plumb through gallium/ to automated documentation

Add Gallium and the Gallium-based drivers to doxygen's automated
code documentation infrastructure.

Can be individually created with:

  cd $MESA_TOP_LEVEL/
  make -C doxygen/ gallium.tag

Benefits from the existing doxygen Makefile runners to clean up
afterwards with 'make clean'.

Signed-off-by: Rhys Kidd <0b3b315d08353e2046d43a86d41e3fd1dff3d6ad@gmail.com>
Reviewed-by: Emil Velikov <f20878473f414da183da19ee25c8722cc1d711e4@collabora.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler