quartz: Try creating the file reader from the filter source first before falling back to the generic async file reader.

quartz: Make mpeg splitter keep track of play position and keep last sample if sending was not successful.

This fixes data discontinuities that earlier occured after a
stop/pause command or the downright rejection of a sample (S_FALSE).

This should fix the problem that in LoadLocation SFLAG_INTEXTURE path gl calls 
can be made without activating context, due to isInDraw checks.