add an important BeginEnder interface

pixelgl/interface.go

@@ -0,0 +1,34 @@
+package pixelgl
+
+// BeginEnder is an interface for manipulating OpenGL state.
+//
+// OpenGL is a state machine and as such, it is natural to manipulate it in a begin-end manner.
+// This interface is intended for all OpenGL objects, that can begin being active and end being active
+// such as windows, vertex arrays, vertex buffers, textures, shaders, pretty much everything.
+//
+// It might seem natural to use BeginEnders this way:
+//
+//   window.Begin()
+//   shader.Begin()
+//   texture.Begin()
+//   vertexarray.Begin()
+//   vertexarray.Draw()
+//   vertexarray.End()
+//   texture.End()
+//   shader.End()
+//   window.End()
+//
+// Don't do this! A better practice is to make a BeginEnder so that it wraps another BeginEnder like this:
+//
+//   shader = NewShader(window)
+//   texture = NewTexture(shader)
+//   vertexarray = NewVertexArray(texture)
+//   // now, somewhere else in your code, instead of calling numerous Begin/Ends, you just calling
+//   vertexarray.Draw()
+//
+// The final single call to draw a vertex array executes all of the Begins and Ends, because the objects are
+// wrapped around each other.
+type BeginEnder interface {
+	Begin()
+	End()
+}