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modesetting: Fix ms_covering_crtc() segfault with non-modesetting slave

ms_covering_crtc() uses RRFirstOutput() to determine a primary output to fall back to if a drawable is overlapping a slave output.

If the primary output is a slave output, RRFirstOutput() will return a slave output even if passed a master ScreenPtr. ms_covering_crtc() dereferences the output's devPrivate, which is invalid for non-modesetting outputs, and can crash.

Changing RRFirstOutput() could have unintended side effects for other callers, so this change replaces the call to RRFirstOutput() with ms_first_output(). ms_first_output() ignores the primary output if it doesn't match the given ScreenPtr, choosing the first connected output instead.

Signed-off-by: Alex Goins

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