Do not clear after allocation in drmsink; fix size type in propose_allocation
Update README
Move graphics-mode property to GstFbdevframebuffersink

Move the graphics-mode property from GstFramebufferSink to
GstFbdevFramebufferSink since it is fbdev/Linux-specific. drmsink
derives directly from GstFramebufferSink but doesn't seem to need
setting the console to graphics mode (there's no interference
from the text console).

GstFramebufferSink is now, in theory, mostly platform-agnostic.
Update README; Properly free screen/overlay buffers in stop function