onscreen/native: Squash adjacent switch statements
Because we can. And it's now clearer that buffer
is only used in META_RENDERER_NATIVE_MODE_GBM
.
Also move next_frame
storage to later in the function. It won't be used until later when we flip, and in fact assigning
it early could have led to its own assertion failing on the next frame
in the unlikely event that we return with "Failed to ensure KMS FB ID...