Limit the number of keystrokes when creating glclear tool

[elgopher]

At the moment developer has to write following code to instantiate glclear tool:

clearTool := glclear.New(gl.Context().NewClearCommand())

Thats a lot. We can simplify this piece of code to something like this:

clearTool := glclear.New(gl.Context())

What is needed is the new interface in glclear package:

type glContext interface {
    NewClearCommand() *gl.ClearCommand
}