[backend-v10] Skip wait condition if vsync timeframe exceeds

The hwcomposer v10 backend can deadlock when setting an ambience theme
in jolla-gallery, as it is waiting for the signal to continue the swapping.
[backend-v10] Only print vsync information once
[backend-v10] Cache VSync FPS in global variable for use in the wait condition
Merge pull request #27 from beidl/master

[backend-v10] Skip wait condition if vsync timeframe exceeds