renderer: Correct description

Correct the description because renderer is an
abstract base class for all renderers, not only
for the console renderer.
renderer: Rename functions

Rename `ClearBuffer()` to `PreRender()` and
`FlushBuffer()` to `PostRender()`.
renderer: Improve documentation
renderer/console: Implement line drawing

Now the console renderer can draw ASCII-lines!
renderer/console: Codestyle improvements

Some codestyle changes according to style +
add dots behind the documentation sentences.
renderer/console: Let DrawPoint() update z-buffer

DrawPoint() only checks whether it should be
rendered because it's in front of the z-buffer,
but doesn't update the z-value if it's really
not covered by other points.
renderer/console: DrawLine() uses now z-buffer

Z-buffer implemented in DrawLine() function.