Add AnchorPos struct and functions

Clipboard Support

#244 GLTriangle's fragment shader is used when rendered by the Canvas.

Use slice for range in Drawer.Dirty(), to improve performance

tests and fixes faiface#123, SIGSEGV on text.NewAtlas if glyph absent

Adding Clipping rectangle support in GLTriangles

Allows for custom fragment shaders to be used after canvas.MakeTriangles.

expose glfw.WaitEvent using Window.WaitInput

Add window hints to created maximized or invisible windows