See the OpenGL ARB enum registry at http://www.opengl.org/registry/api/enum.spec
Blocks allocated to Mesa:
0x8750-0x875F
0x8BB0-0x8BBF
GL_MESA_packed_depth_stencil
GL_DEPTH_STENCIL_MESA 0x8750
GL_UNSIGNED_INT_24_8_MESA 0x8751
GL_UNSIGNED_INT_8_24_REV_MESA 0x8752
GL_UNSIGNED_SHORT_15_1_MESA 0x8753
GL_UNSIGNED_SHORT_1_15_REV_MESA 0x8754
GL_MESA_trace.spec:
GL_TRACE_ALL_BITS_MESA 0xFFFF
GL_TRACE_OPERATIONS_BIT_MESA 0x0001
GL_TRACE_PRIMITIVES_BIT_MESA 0x0002
GL_TRACE_ARRAYS_BIT_MESA 0x0004
GL_TRACE_TEXTURES_BIT_MESA 0x0008
GL_TRACE_PIXELS_BIT_MESA 0x0010
GL_TRACE_ERRORS_BIT_MESA 0x0020
GL_TRACE_MASK_MESA 0x8755
GL_TRACE_NAME_MESA 0x8756
MESA_ycbcr_texture.spec:
GL_YCBCR_MESA 0x8757
GL_UNSIGNED_SHORT_8_8_MESA 0x85BA /* same as Apple's */
GL_UNSIGNED_SHORT_8_8_REV_MESA 0x85BB /* same as Apple's */
GL_MESA_pack_invert.spec
GL_PACK_INVERT_MESA 0x8758
GL_MESA_shader_debug.spec: (obsolete)
GL_DEBUG_OBJECT_MESA 0x8759
GL_DEBUG_PRINT_MESA 0x875A
GL_DEBUG_ASSERT_MESA 0x875B
GL_MESA_program_debug.spec: (obsolete)
GL_FRAGMENT_PROGRAM_CALLBACK_MESA 0x????
GL_VERTEX_PROGRAM_CALLBACK_MESA 0x????
GL_FRAGMENT_PROGRAM_POSITION_MESA 0x????
GL_VERTEX_PROGRAM_POSITION_MESA 0x????
GL_FRAGMENT_PROGRAM_CALLBACK_FUNC_MESA 0x????
GL_FRAGMENT_PROGRAM_CALLBACK_DATA_MESA 0x????
GL_VERTEX_PROGRAM_CALLBACK_FUNC_MESA 0x????
GL_VERTEX_PROGRAM_CALLBACK_DATA_MESA 0x????
GL_MESAX_texture_stack:
GL_TEXTURE_1D_STACK_MESAX 0x8759
GL_TEXTURE_2D_STACK_MESAX 0x875A
GL_PROXY_TEXTURE_1D_STACK_MESAX 0x875B
GL_PROXY_TEXTURE_2D_STACK_MESAX 0x875C
GL_TEXTURE_1D_STACK_BINDING_MESAX 0x875D
GL_TEXTURE_2D_STACK_BINDING_MESAX 0x875E