16-bit color accumulation target and scissor rects

Rendering point lights onto flat surfaces with no texturing leads to annoying quantization rings that are very visible when only using 8-bits per color. This problem is even worse when two point lights cover the same area, which makes a diamond shaped pattern as in the included screenshot:

In order to help combat this, I changed the forward and basic deferred rendering path to perform the color accumulation from lighting into a 16-bit per channel texture. While this doesn't get rid of the ring effect from a single point light, it does fix the diamond pattern when two point lights intersect. Also, rendering into a texture is better anyway because it is what I will need to do when I start working on post processing effects; plus, if I want to actually perform some sort of HDR scaling to the accumulated colors, this will make it easy to do so.

I also implemented a seemingly correct scissoring rectangle calculation for point lights. It speeds up the deferred shading render by a significant amount depending on the scene and the size of the point lights. On the simple scene (spiders + monkey head + cuboids), the deferred shader is twice as fast as the forward renderer with moderate sized point lights. As the point lights get smaller, this gap in performance increases, with the forward render staying constant and the deferred renderer getting faster. I've attached a picture showing giving a rough view of the scissoring rectangles:

The next step will be to write the tile-based light calculation using CUDA.