gpu: expose the rendering implementation

The rendering implementation is needed for using Gio UI with external
window libraries such as GLFW. Expose it in the new package gpu.

Updates #26

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

gpu/gl/types.go

@@ -0,0 +1,35 @@
+// +build !js
+
+package gl
+
+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
+}
+
+func (t Texture) Valid() bool {
+	return t.V != 0
+}
+
+func (t Texture) Equal(t2 Texture) bool {
+	return t == t2
+}