rfbsrc: Implement decide_allocation virtual

rfbsrc: Implement decide_allocation virtual

This way we can use the base class for buffer allocation, hence use
fill() instead of create() virtual. This also adds a strict check on the
select pool buffer size as we don't support strides and padding.

This is based on initial patch proposed by Sebastien Dröge, from which I
also fixed a buffer pool leak.

https://bugzilla.gnome.org/show_bug.cgi?id=763441