Fill and texture func

[schell]

• moved shader loading int Gelatin/GL/Shaders.hs
• moved primitive proxying stuff from Gelatin/GL.hs int Gelatin/GL/Primitives.hs
• added new fill hashing technique
• added filled (color or texture) text
• removed the FontClass typeclass, replaced with FontData record
• sdl2 backend has better dpi calculation and has actual framebuffer size calculation
• exposed window configuration for sdl2 backend
• added stack.yaml's and .sosrc's

[schell]

This should fix #4