Fix hash map collisions when setting canvases.

src/modules/graphics/opengl/Canvas.cpp

@@ -20,6 +20,7 @@
 
 #include "Canvas.h"
 #include "graphics/Graphics.h"
+#include "Graphics.h"
 
 #include <algorithm> // For min/max
 
@@ -270,6 +271,12 @@ bool Canvas::loadVolatile()
 
 void Canvas::unloadVolatile()
 {
+	// This is a bit ugly, but we need some way to destroy the cached FBO
+	// when this Canvas' texture is destroyed.
+	auto gfx = Module::getInstance<Graphics>(Module::M_GRAPHICS);
+	if (gfx != nullptr)
+		gfx->cleanupCanvas(this);
+
 	if (fbo != 0)
 		gl.deleteFramebuffer(fbo);