Wayland, macOS & SDL weren't using the start-time of the process.
The start-time offset isn't needed so it's similar to forward the
systems method of accessing the time in milliseconds.
This adds a new Ghost function, GHOST_GetPixelAtCursor, that allows
picking colors from outside of Blender windows. This only has an
implementation for the Windows platform, but this should allow other
platforms to also do so if possible.
Pull Request: https://projects.blender.org/blender/blender/pulls/105324
* opengl_context -> system_gpu_context. This is the operating system OpenGL,
Metal or Vulkan context provided by GHOST.
* gpu_context -> blender_gpu_context. This is the GPUContext provided by
the Blender GPU module, which wraps the GHOST context and adds some state.
* Various functions create/destroy/enable/disable both contexts, these have
just gpu_context in the name now.
Pull Request: https://projects.blender.org/blender/blender/pulls/108723
