Bitmap: Remove leftover 'flush()' calls
MRuby-Binding: Fix etc accessors not using getter/setter functions
Input: Implement RGSS3 functionality in bindings

Ie. using symbols instead of Input:: constants to query
button states.
mkxp.pro: Add RGSS3 config define
Sprite: Implement wave effect (RGSS2)

This initial implementation emulates the way RMVX splits
the sprite into "chunks" of about 8 pixels, which it then
scrolls left/right on a vertical sine wave. It even
replicates the weird behavior when wave_amp < 0, namely
"shrinking" the src_rect horizontally.

As with bush_opacity, this effect in combination with
rotation will render differently from RMVX.