SetupForBlit sets up the GL viewport and projection matrix for
screen-cordinate access to the framebuffer. These settings were not
updated if the other gl states were already set up for blitting. Guild
Wars reads back an offscreen rendered texture from the framebuffer,
which currently sets up CTXUSAGE_BLIT, then changes the render target,
and draws to the texture, which has to be reloaded from system memory
before it can be rendered to(since GW loaded some data into it). If the
two render targets had different size this failed.