streamlined api

* clearer and simpler variable names, like min/max instead of
m_min/m_max
* Circle::contains
* more methods for g_screen (like setColor, draw* and printf), to make
it more like a graphics context
* improved plane-drawing to support optional grids
* operator-overloaded math is standard now (instead of nominal math
methods)
* used better raytracing math for circle logic in v2.h