Don't unnecessarily expose internal constant
mkxp.pro: Add 'BOOST_LIB_SUFFIX'

Boost being boost.
GLState: Get rid of GL_TEXTURE_2D enable/disable

This bit was deprecated/removed in core GL.

There was only one place where this was used (flash tiles
in Tilemap), and since the full shader rewrite, it was
effectively a no-op anyway (flash shader doesn't sample texture).