r_brush.c: small optimization in R_DrawSequentialPoly; move the drawing of fullbrights to a separate function, and draw them in a separate pass through the polys of the brush mode. This reduces the amount of texture binding. The extreme case would be, e.g. 1000 polys with one texture (with fullbrights), before would require 2000 binds, after would require 2 binds.  This gets part way to the performance of https://sourceforge.net/p/quakespasm/patches/14/ with much less code change

git-svn-id: svn://svn.code.sf.net/p/quakespasm/code/trunk/quakespasm@968 af15c1b1-3010-417e-b628-4374ebc0bcbd