Primary render fallback

[cmeissl]

This adds support for display only nodes by falling back to the primary gpu for rendering.
In addition to the render fallback it also tries to import foreign gbm buffers through dmabuf export/import.

Unfortunately it seems the DisplayLink driver has some issues regarding vsync and will issue a vblank right after
scheduling a page flip. This leads to visual tearing, probably caused by the user-space driver part. As a mitigation this PR also adds a way to throttle vblanks.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 6.83%. Comparing base (2cf66ca) to head (e7bf5dc).
Report is 1 commits behind head on master.

❗ There is a different number of reports uploaded between BASE (2cf66ca) and HEAD (e7bf5dc). Click for more details.

HEAD has 3 uploads less than BASE

Flag	BASE (2cf66ca)	HEAD (e7bf5dc)
wlcs-buffer	1	0
wlcs-core	1	0
wlcs-pointer-input	1	0

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #1680       +/-   ##
==========================================
- Coverage   19.17%   6.83%   -12.35%     
==========================================
  Files         175     175               
  Lines       28562   28569        +7     
==========================================
- Hits         5478    1953     -3525     
- Misses      23084   26616     +3532     

Flag	Coverage Δ
wlcs-buffer	?
wlcs-core	?
wlcs-output	6.83% <ø> (-0.01%)	⬇️
wlcs-pointer-input	?

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[Drakulix]

Looks great!