Reference images in the reference_override_dir will be chosen before
images in reference_dir. This allows platform specific reference
images, with a common base.
Ignored when set to None. The caller is responsible
of setting the reference override dir as the unit test is more aware
what the definition of a platform is.
Patch adds `gpu.platform.device_type_get` function to get the device
type that blender has detected.
Reviewed By: brecht
Maniphest Tasks: T99046
Differential Revision: https://developer.blender.org/D15265
