Cleanup Rasterization Code (#592)

Mostly just improving variable names for clarity.

Removed a redundant base-case check in AddSpan that resulted in a small but measurable perf improvement due to removing a branch. Also tightened up the bounding box overlap check so it early-outs faster, which also has a small perf improvement to rasterization times.

Updated docstrings for user-facing functions.

* Moved docstring comments to the header with the rest of the docstring
* Removed unused function
* Make a cpp-only function static
* Added docstring for rasterizeTri
* Use spaces to indent function parameter lists so they look good regardless of the editor's tab width
* Better parameter names for rcClamp
* Cleanup code formatting, a couple variable names, and some comments in rasterizeTri
* Clean up dividePoly, added rcAxis enum
* Cleanup rasterizeTri docstring
* Faster overlapBounds check in rasterization
* Renamed some vars in rasterizeTri
* cleanup addSpan and remove redundant branch
* addSpan docstring
* Cleanup the implementations of rcRasterizeTriangle and rcRasterizeTriangles overloads
* Added docstrings and renamed vars in rcAllocSpan and rcFreeSpan
* Clean up rcRasterizeTriangle and rcRasterizeTriangles in the header.  Removed redundant const qualifiers on value parameters
* Better parameter names for rasterizeTri, updated docstring, fixed some incorrect comments
* Improved parameter names of rcRasterizeTriangle, rcRasterizeTriangles, and rcAddSpan
* Fixed some old variable names referenced in the docstring for rcAddSpan