3D mesh painter

• refactor duplicate code from mesh-render/bake-render shaders
• keep from painting on one panel if other dirty
• remove floating gl* references, use inheritance instead
• optimization: create paint FBO only when needed
• show correct paint color for overlay brush
• fix "blurring" that happens to paint buffer on brush drags
• redirect key events to correct widget
• clean up texture/mesh ownership
• fix OpenGL context sharing across app
• swap Point2 typedef for QPointF
• add anti-aliasing to viewport
• bake around edges of UV view to improve texture filtering