WIP: Virglenderer unstable APIs and CONTEXT_INIT feature
Use a combo option for virglrenderer to be able to choose between enabled, disabled, auto, and unstable. The latter is useful for developers which might want to test virglrenderer's unstable APIs.
Create virgl renderer context with flags using context_id when valid.
v2: The feature can be enabled via the context_init config option. v3: A warning message will be emitted and the feature will not be used when linking with virglrenderer versions without context_init support.