renamed most Image_CopyMux parameters to begin with input to clarify their purpose...

renamed most Image_CopyMux parameters to begin with input to clarify their purpose, inputwidth for example, because these are input properties, not output properties
cleaned up Image_CopyMux, and fixed its handling of inputflipdiagonal to properly work with unequal inputwidth/inputheight
swapped Image_CopyMux numinputcomponents/numoutputcomponents parameters to be more consistent with the 'output, input' order of its other parameters

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@4168 d7cf8633-e32d-0410-b094-e92efae38249