Implements disabled selections.

We now obey options in the select() command to disable certain options. These
are presented with a different item background in the case of SELBTNs.

This also makes the select buttons in Little Busters look closer to the
official interpreter.
Implement composite mode 2.

In Clannad, there are sliding clickable targets during the Tomoyo vs.
Sunohara scenes which rely on a combination of objColour() and
objComposite(). This never totally worked, but was at least visually
acceptable until I switched to the object shader. Fix the blending.

Closes GH-46
Implement music fade in, and use it for all music play operations.

rlvm has had a longstanding issue where there would be a click/pop
whenever the background music track would start playing. This
implements the music fade in command (which we didn't previously
support), and then makes all plays fade in over 50ms, hiding the
pop.
Fix multiline string objects.

The rldev documentation states that the fifth parameter to objTextOps()
is "vertical," where if it is 1, the text is vertical. This is actually
the number of characters that should be placed on the line, with 0
being a magic value.

This fixed display of several options in Little Busters.