Fixed a bug in the line drawing algo
Added function to pixmap.h that allows drawing lines via polar coordinates (rather than point-to-point)
Pixmap's members height and width are now pixmap_height and pixmap_width, plus misc refactoring
Added support for drawing filled circles
Added ability to fill an area around a point and some code refactoring
Overloaded the += operator for the Pixel type and fixed an invalid read in Pixmap::fill_area()
Overloaded the -= and == operators for the Pixel type
Removed  private function Pixmap::equals in favour of overloaded operator == for Pixel type
Added a test module for constructing various test images, and misc. cleanup
Added support for additive overlay for circles
Added support for additive overlay when drawing squares and rectangles; added a test image for those cases