gpu/gl: remove OpenGL functions parameter from NewBackend

As a consequence, most API is gone from gpu/gl, and embedding Gio in
foreign frameworks don't need to provide an OpenGL implementation.

The next change simplifies the GLFW embedding example accordingly.

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

internal/glimpl/types.go

@@ -0,0 +1,27 @@
+// +build !js
+
+package glimpl
+
+type (
+	Buffer       struct{ V uint }
+	Framebuffer  struct{ V uint }
+	Program      struct{ V uint }
+	Renderbuffer struct{ V uint }
+	Shader       struct{ V uint }
+	Texture      struct{ V uint }
+	Query        struct{ V uint }
+	Uniform      struct{ V int }
+	Object       struct{ V uint }
+)
+
+func (u Uniform) Valid() bool {
+	return u.V != -1
+}
+
+func (p Program) Valid() bool {
+	return p.V != 0
+}
+
+func (s Shader) Valid() bool {
+	return s.V != 0
+}