mpegvideo: cosmetics: Lowercase ugly uppercase MPV_ function name prefixes
build: Add explanatory comments to (optimization) blocks in the Makefiles
cosmetics: Write NULL pointer equality checks more compactly

Signed-off-by: Diego Biurrun <diego@biurrun.de>
cosmetics: Write NULL pointer inequality checks more compactly

Signed-off-by: Diego Biurrun <diego@biurrun.de>