OpenGL always offers filtering on all formats, and if the hardware
doesn't support it the driver falls back to software. Direct3D on the
other hand silently disables filtering, so that's what we should do too.

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.