Turns RenderLine line branches into template parameters, allowing the
compiler to eliminate the branches and also fully inline it.
Example FPS change
* In dungeon: 1450 -> 1530
* In town: 1655 -> 1700
Also splits RenderLine into 3 functions
Easier to read and also gives more useful profiling.
Apparently the most time is spent in `RenderLineOpaque`.
Also, mark them `inline` because that apparently hints GCC to inline
the function (in a later refactoring we can introduce attribute
always_inline instead where supported).