- Deduplicate rendering driver names.
- Tweak grammar and punctuation to be consistent across both messages.
- Use "rendering method" terminology in the relevant error message.
Preview:
`--rendering-driver unknown`:
Unknown rendering driver 'unknown', aborting.
Valid options are 'vulkan', 'opengl3', 'opengl3_es' and 'dummy'.
`--rendering-method unknown`:
Unknown rendering method 'unknown', aborting.
Valid options are 'forward_plus', 'mobile' and 'gl_compatibility'.