This implements different blend modes in the SoftwareRasterizer by
first setting up the blend factors then rendering the pixels into a
temporary buffer and finally mixing the contents of the temporary buffer
with the contents of the backbuffer based on the blend factors.
This untangles several concepts in the rasterizer and makes it possible
to toggle different stages on a per-block level rather than having to
check whether the feature is enabled for every pixel.
This makes the software rasterizer use integers for triangle coverage
calculations. The previously used floating point algorithm was not
precise enough in certain situations and showed gaps between triangles.
This is not yet subpixel accurate.
Tests against and writes to the depth buffer when GL_DEPTH_TEST is
enabled via glEnable(). Currently fragment z is always compared against
existing depth with GL_LESS.
This is based mostly on Fabian "ryg" Giesen's 2011 blog series
"A trip through the Graphics Pipeline" and Scratchapixel's
"Rasterization: a Practical Implementation".
The rasterizer processes triangles in grid aligned 16x16 pixel blocks,
calculates barycentric coordinates and edge derivatives and interpolates
bilinearly across each block.
This will theoretically allow for better utilization of modern processor
features such as SMT and SIMD, as opposed to a classic scanline based
triangle rasterizer.
This serves as a starting point to get something on the screen.
In the future we might look into properly pipelining the main loop to
make the rasterizer more flexible, enabling us to enable/disable
certain features at the block rather than the pixel level.
