graphicsdriver/opengl: Experimental PBO implementation

This change is an experimental implementation to use Pixel Buffer
Objects. This reduces calls of glTexSubImage2D.

This works only on desktops. Unfortunately WebGL does not have
this features. Mobiles can have PBO as of OpenGL ES 3.

Updates #976

internal/graphicsdriver/opengl/context_js.go

@@ -470,3 +470,7 @@ func (c *context) flush() {
 func (c *context) needsRestoring() bool {
 	return !web.IsMobileBrowser()
 }
+
+func (c *context) canUsePBO() bool {
+	return false
+}