changed r_textshadow behavior to always draw a black shadow, but with

changed r_textshadow behavior to always draw a black shadow, but with
alpha dependent on the brightness of the primary text color, this means
you don't get white shadows under black text, but do get black shadows
under white text
rewrote DrawQ_String and friends to be one combined function that draws
colored strings more efficiently by batching characters with differing
colors together, and it even batches the shadow with the text which can
help performance significantly with r_textshadow on

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@7119 d7cf8633-e32d-0410-b094-e92efae38249