Rename colored.vertex.glsl → default.vertex.glsl
render_system: use own camera for drawing cameras

Previously we reused the default 2D camera of the window being drawn,
but users can modify its settings. We need to use a seperate camera to
ensure that the settings are correct.
camera: add pixels_to_units property

Sets the number of pixels that one world space unit represents.
Applies only to orthographic cameras. Ignored in perspective cameras.