Text: improve AbstractFont documentation.
Text: test line advance behavior when rendering scaled text.

Currently the test fails, because line advance doesn't take text size
into account.
Text: fix rendering of scaled multi-line text.

Take text size into account when advancing lines. I would probably not
run into this issue if I would visually test this on anything else than
unscaled pixel art font.
Text: minor cleanup.
Text: harden and clean up Renderer test.
Text: test that proper index type is used in Renderer.

Something smells fishy here.
Text: it's totally fine to have 256 vertices indexed with UnsignedByte.
Text: fix array index type in Renderer.

When we have 256 vertices and >256 indices, we need more than 8 bits to
index the index array. The tests are passing again.