Fixed surfaceSTToTexelUV_bilinearQuad()’s calculation

• Fixed surfaceSTToTexelUV_bilinearQuad() to return the pointUVs-lerped UV coordinate, instead of (mistakenly) the just the textureST coordinates.
	» While mathematically this is the same as just taking the previous result and lerping it against the pointUVs, in practice we were able to calculate a couple of pointUV intermediates along the way and remove a unneeded ST calculation.
	» This fixes the 1st/3rd-divide-mirroring issue.  Hooray!