Stop using System.Drawing

[opcon]

The System.Drawing namespace always causes problems with Mono, and while QuickFont works okay now, it is a good idea to move away from System.Drawing altogether.

The following is a list of ways System.Drawing is used in the current codebase, and a replacement:

Use	Alternative
Bitmap manipulation	ImageSharp, a cross-platform image manipulation library. Still in early alpha, but probably usable enough.
Size/Rectangle/Point structs	Can just implement these as standalone structs - see OpenTK
Text Rendering hints for bitmaps	Not sure if this is needed or will make a difference
Converting SharpFont FTBitmaps to GDIBitmaps	Can replace this conversion mechanism with one using ImageSharp
Loading fonts in the GDIFont class	No alternative, this method of loading uses System.Drawing specifically for loading the fonts and so will be removed along with System.Drawing
Color class/list	OpenTK has Color4, could use that instead. What does ImageSharp use?
Font Style	Can create a simple enumeration for this
Brushes	Can probably use ImageSharp

[opcon]

ImageSharp contains the Size, Rectangle(F), Point structures, as well as an equivalent of the System.Drawing.Color struct. There is also some form of brush support.