Add CMake flag FORCE_OPENGL_ENABLED

[clintlombard]

This allows building Colmap with GUI_ENABLED=OFF, but still allowing OpenGL support.

Related to #1431

[clintlombard]

I may be mistaken, but having these here doesn't make sense because GL_LUMINANCE and GL_UNSIGNED_BYTE in ExtractSiftFeaturesGPU won't be defined. This causes compilation errors.

[ahojnnes]

Thanks and apologies for the slow response. I think, the change looks good, but I would suggest to revert the option and call it DISABLE_OPENGL, which would be off by default?

[RemiLacroix-IDRIS]

Note that OpenGL handling is generally a bit weird. The option says Whether to enable OpenGL, if available but the package is always flagged as required in CMakeLists.txt: find_package(OpenGL REQUIRED). It means you actually cannot compile if OpenGL (and glew) are not found.

[kuuhakuJ]

thx, it works!