player: can't render video properly with libmpv(0.38) on Linux

[cosven]

from libmpv API changelog

--- mpv 0.38.0 ---
2.3 - partially revert the changes from API version 1.27, remove libmpv as
the default VO and move it to the bottom of the auto-probing order.
This restores the prior behavior on all platforms other than macOS,
but still auto selects libmpv/cocoa-cb on macOS if it was built with
support for cocoa-cb.

so we should set libmpv as vo explicitly.

[coveralls]

Pull Request Test Coverage Report for Build 8771834576

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.003%) to 55.271%

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Totals
Change from base Build 8770800639:	0.003%
Covered Lines:	9725
Relevant Lines:	17595

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💛 - Coveralls