The renderer can now update any combination of a Drawable's position,
direction, scale, and effect values through a single function call. The
effect values will be adjusted according to per-effect conversion
functions to prepare the values for use inside the shader.
This change introduces the Drawable class, which corresponds to a
Scratch sprite or clone. It supports setting its "skin" (corresponding
to a Scratch costume) by md5+extension, but currently only supports
bitmap skins. Drawables can be created, destroyed, and otherwise
manipulated by ID through the renderer.
