MPEGPlayer: Try out a different frame drop scheme meant to skip in a more uniform...

MPEGPlayer: Try out a different frame drop scheme meant to skip in a more uniform way rather than running up late and jumping forward; will often drop more in long term to keep up in short term. Some other obscure fixes included: wait for 2 ref pics before decoding B-pics again after P or I frame drop or seeking (issue with open GOPs); draw the frame the decoder already has when beginning playback after a seek; rename a few vars.

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@29198 a1c6a512-1295-4272-9138-f99709370657