video: Fix errors in calculation of transform

video: Fix errors in calculation of transform

It was discovered that change of video wasn't able to be applied with
the state, which was destination area out bound of screen with
WL_OUTPUT_TRANSFORM_90.

It turned out because of errors in calculation of transform. That error
made source(input) area empty and empty area cannot be applied.

The transform value which caused this issue was as follow:

wl_subsurface_set_position(840, 0)
wl_viewport_set_destination(1080, 1920)
wl_viewport_set_source(0, 1936, 1080, 2160)
wl_surface_set_buffer_transform(WL_OUTPUT_TRANSFORM_90)

* The state of output: size (1920x1080) transform (0)

Change-Id: I3134980d46305a03965076c843a568fd84f226d0