This PR allows blender to be compiled with OpenXR, but without OpenGL support. For investigating a bug we needed to compile Blender without OpenGL (`WITH_OPENGL_BACKEND=OFF`) but it would fail to compile OpenXR (`WITH_XR_OPENXR=ON`). This PR tweakes the preprocess directives to compile the openxr graphics binding without OpenGL. Co-authored-by: Jonas Holzman <jonas@holzman.fr> Pull Request: https://projects.blender.org/blender/blender/pulls/141668