separation of previous renderer into base and derivation complete

The previous iteration was fairly OpenGL-heavy. It's now been separated into
a base class that is OpenGL independent and a derived class that is
OpenGL-based. Over time this should allow for portability away from OpenGL
if necessary.

app/render/backend/opengl/openglframebuffer.cpp

@@ -88,7 +88,7 @@ void OpenGLFramebuffer::Release()
   context_->functions()->glBindFramebuffer(GL_DRAW_FRAMEBUFFER, 0);
 }
 
-void OpenGLFramebuffer::Attach(RenderTexturePtr texture)
+void OpenGLFramebuffer::Attach(OpenGLTexturePtr texture)
 {
   if (context_ == nullptr) {
     return;
@@ -98,7 +98,7 @@ void OpenGLFramebuffer::Attach(RenderTexturePtr texture)
   AttachInternal(texture_->texture(), false);
 }
 
-void OpenGLFramebuffer::AttachBackBuffer(RenderTexturePtr texture)
+void OpenGLFramebuffer::AttachBackBuffer(OpenGLTexturePtr texture)
 {
   if (context_ == nullptr) {
     return;