gpu/internal/opengl: restore BeginFrame state in EndFrame

To ease the integration with foreign OpenGL contexts, carefully save the
context state before rendering a frame and restore it afterwards. Gio
rendering can then be mixed with OpenGL code that expects exclusive
control over context state.

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

internal/gl/types.go

@@ -3,20 +3,28 @@
 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 }
-	VertexArray  struct{ V uint }
 	Object       struct{ V uint }
+	Buffer       Object
+	Framebuffer  Object
+	Program      Object
+	Renderbuffer Object
+	Shader       Object
+	Texture      Object
+	Query        Object
+	Uniform      struct{ V int }
+	VertexArray  Object
 )
 
+func (o Object) valid() bool {
+	return o.V != 0
+}
+
+func (o Object) equal(o2 Object) bool {
+	return o == o2
+}
+
 func (u Framebuffer) Valid() bool {
-	return u.V != 0
+	return Object(u).valid()
 }
 
 func (u Uniform) Valid() bool {
@@ -24,13 +32,45 @@ func (u Uniform) Valid() bool {
 }
 
 func (p Program) Valid() bool {
-	return p.V != 0
+	return Object(p).valid()
 }
 
 func (s Shader) Valid() bool {
-	return s.V != 0
+	return Object(s).valid()
 }
 
 func (a VertexArray) Valid() bool {
-	return a.V != 0
+	return Object(a).valid()
+}
+
+func (f Framebuffer) Equal(f2 Framebuffer) bool {
+	return Object(f).equal(Object(f2))
+}
+
+func (p Program) Equal(p2 Program) bool {
+	return Object(p).equal(Object(p2))
+}
+
+func (s Shader) Equal(s2 Shader) bool {
+	return Object(s).equal(Object(s2))
+}
+
+func (u Uniform) Equal(u2 Uniform) bool {
+	return u == u2
+}
+
+func (a VertexArray) Equal(a2 VertexArray) bool {
+	return Object(a).equal(Object(a2))
+}
+
+func (r Renderbuffer) Equal(r2 Renderbuffer) bool {
+	return Object(r).equal(Object(r2))
+}
+
+func (t Texture) Equal(t2 Texture) bool {
+	return Object(t).equal(Object(t2))
+}
+
+func (b Buffer) Equal(b2 Buffer) bool {
+	return Object(b).equal(Object(b2))
 }