Stereo 3D|Fixed: Blacked out floors and other surfaces

The alpha test was still on, preventing some surfaces from being
drawn to the framebuffer via the Oculus Rift distortion shader.

The default GL state of the legacy renderer has alpha test enabled.

doomsday/client/src/ui/vrcontenttransform.cpp

@@ -132,20 +132,25 @@ DENG2_PIMPL(VRContentTransform)
 
         GLState::pop().apply();
 
-        glEnable(GL_TEXTURE_2D); // Necessary until the legacy code uses GLState, too.
+        // Necessary until the legacy code uses GLState, too:
+        glDisable(GL_ALPHA_TEST);
+        glEnable(GL_TEXTURE_2D);
 
         // Return the drawing to the full target.
-        GLState::setActiveRect(Rectangleui()/*, true*/);
+        GLState::setActiveRect(Rectangleui(), true);
 
         canvas().renderTarget().clear(GLTarget::Color);
         GLState::push()
                 .setBlend(false)
                 .setDepthTest(false);
 
+        glDisable(GL_BLEND);
+
         // Copy contents of offscreen buffer to normal screen.
         oculusRift.draw();
 
         glBindTexture(GL_TEXTURE_2D, 0);
+        glEnable(GL_ALPHA_TEST);
         glDepthMask(GL_TRUE);
 
         GLState::pop().apply();