Convert all coordinates within the visual tools to Vector2D, which has
been significantly extended. Eliminates a lot of issues with accumulated
rounding errors and simplifies a lot of code.
Modernize the visual tools' interactions with the rest of Aegisub by
connecting to signals directly rather than routing everything through
the video display and converting the main visual tool mode toolbar to
the command system.
Extract all references to OpenGL from the visual tools and move them to
OpenGLWrapper as a first step towards making it possible to implement an
alternative video renderer. In the process, eliminate all uses of OpenGL
immediate mode.
Fix a bunch of minor issues and general instability.
Originally committed to SVN as r5823.
