Removed RenderingContextId param. from the rendering methods.
Dropped the fixed pipeline matrices for wires.
Added Projection2D class to replace the fixed legacy matrix stack.
	renamed:    main/knobs.cpp -> main/Knob.cpp
	renamed:    main/knobs.h -> main/Knob.h
Projection2D can now project points =)
Not deleting active RenderingContextAware items. This caused a segfault.
Using Projection2D.

Knobs are broken for now.
Added new painter that can draw arbitrary textured shapes.
Added some new icons.
read_png() is now inline.
Checking for gl errors.

Added a handy CHECK_FOR_GL_ERRORS macro that works in DEBUG mode.
Cheking for gl errors in a lot of places.
Passing proper values for sampler uniforms.
Dropped some old fixed-pipeline code.
Some other minor changes.
Playing with mouse cursors.
Preparing to move to the new painter.