app/internal/gl/impl: extract platform dependent opengl bindings

We'd like to support Gio using a different renderer binding than
the builtin. A first step is to define the Functions interface
in package gl, and extract the concrete implementations to a
separate package.

Updates #26

Signed-off-by: Elias Naur <mail@eliasnaur.com>

app/internal/gpu/context.go

@@ -11,7 +11,7 @@ import (
 
 type context struct {
 	caps caps
-	*gl.Functions
+	gl.Functions
 }
 
 type caps struct {
@@ -32,7 +32,7 @@ type textureTriple struct {
 	typ            gl.Enum
 }
 
-func newContext(glctx *gl.Functions) (*context, error) {
+func newContext(glctx gl.Functions) (*context, error) {
 	ctx := &context{
 		Functions: glctx,
 	}