screencasting: generate the composed textures with the proper size

Don't neglect the scale of the buffer for windows that we are rendering
on the spot.
Otherwise we will be offering a smaller size to what we are generally
using. More importantly, the ScreencastManager expects the buffers
scaled. This causes glitches when the stream starts otherwise as the
first frame triggers a resize.

CCBUG: 428594


(cherry picked from commit 92c00d1d)