Rename a few QuadShader bits to better match IndexedTrisShader

My brain is getting increasingly angry every time I bounce between
the two files things are ALMOST the same but not quite.
Just qualify those instead of statically importing everything.

Another bit that QuadShader did oh-so-slightly differently from
GLShader/IndexedTrisShader.
De-privatize some bits so that subclasses can add data.

You can now extend vec4sPerQuad to get your own vec4s on the
data array to do with as you please.

A couple notes while they're fresh in my head:
 * You might add something directly as its own uniform or attribute instead
   of just adding it to u_Data. If you do that, you'll need to override
   extraVec4s() so that appropriate space is left for them.
 * You'll potentially want to write your own version of isLikelyToPerform to
   decide whether to use your new shader since things might be close enough
   to the wire that your extra data makes you unable to push enough quads to
   be worth it. It's not completely critical, though: isLikelytoPerform ensures
   room for 16 quads, so it's unlikely that you'll fatten things up so badly
   that it drops TOO far below that.
Break out QuadShader's vertex shader bits ala IndexedTrisShader.

There's a lot of frustratingly ALMOST identical bits between the two
(e.g. VERT_SETCOLOR, VERT_SETPOS) that I'm really tempted to pull
out to a common place, but it's probably not quite worth the pain.