This fixes viewport compositing, when the requested texture size doesn't match the viewport size. Tested for regular viewport and offscreen rendering.
Its not a very elegant, but is at least is localized in one place.
Note, that I'm not running a graphics card with real missing npot support, I just had to fake it by setting GPUGlobal.npotdisabled = 1; at startup.
Also, OpenGL2.0 from 2005 supports npot, so am wondering how long we should be supporting this for (or if hardware without npot would be capable of GPU compositing).