Error message fix.
Canvas documentation.
Added test for negative canvas size.
Throw error if canvas size is negative.
Merge pull request #545 from libavg/bugfix/544_negative_canvas_size

Bugfix/544 negative canvas size
Include cleanup.
Include cleanup.
Include cleanup.
Remove duplicate definition of pi.
Include cleanup.
Linux build fix.
Include cleanup.
Include cleanup.
Include cleanup, cosmetics.
Include cleanup (src/graphics).
Include cleanup (src/graphics).
Include cleanup (src/graphics).
Include cleanup (src/graphics).
Include cleanup (src/graphics).
Include cleanup (src/video).