Added MRT support to canvases via love.graphics.setCanvases(c1, c2, ...), and an 'effects' callback function in shaders.

- Simultanious rendering to multiple canvases requires all canvases to be the same size (limitation of pre-GL3.0 framebuffers)
- love.graphics.isSupported("mrtcanvas") was added
- love_Canvases[n] (0-based array of active canvases) can be written to in pixel shaders in the 'effects' callback function
- everything drawn to multiple canvases at once will be duplicated to all canvases if no shader is active or the standard 'effect' shader function is used

--HG--
branch : MRTs

src/modules/graphics/opengl/Canvas.h

@@ -21,6 +21,7 @@
 #ifndef LOVE_GRAPHICS_OPENGL_CANVAS_H
 #define LOVE_GRAPHICS_OPENGL_CANVAS_H
 
+// LOVE
 #include "graphics/DrawQable.h"
 #include "graphics/Volatile.h"
 #include "graphics/Image.h"
@@ -31,6 +32,9 @@
 #include "common/Matrix.h"
 #include "OpenGL.h"
 
+// STL
+#include <vector>
+
 namespace love
 {
 namespace graphics
@@ -50,7 +54,11 @@ public:
 	Canvas(int width, int height, TextureType texture_type = TYPE_NORMAL);
 	virtual ~Canvas();
 
-	void startGrab();
+	/**
+	 * @param canvases A list of other canvases to temporarily attach to this one,
+	 * to allow drawing to multiple canvases at once.
+	 **/
+	void startGrab(const std::vector<Canvas *> &canvases);
 	void stopGrab();
 
 	void clear(const Color &c);
@@ -66,6 +74,8 @@ public:
 
 	void getPixel(unsigned char* pixel_rgba, int x, int y);
 
+	const std::vector<Canvas *> &getAttachedCanvases() const;
+
 	void setFilter(const Image::Filter &f);
 	Image::Filter getFilter() const;
 
@@ -89,20 +99,22 @@ public:
 	void unloadVolatile();
 
 	static bool isSupported();
-	static bool isHdrSupported();
+	static bool isHDRSupported();
+	static bool isMRTSupported();
 	static bool getConstant(const char *in, TextureType &out);
 	static bool getConstant(TextureType in, const char *&out);
 
 	static Canvas *current;
 	static void bindDefaultCanvas();
 
-private:
-	friend class Shader;
 	GLuint getTextureName() const
 	{
 		return img;
 	}
 
+private:
+	friend class Shader;
+
 	GLsizei width;
 	GLsizei height;
 	GLuint fbo;
@@ -121,6 +133,9 @@ private:
 		Image::Wrap   wrap;
 	} settings;
 
+	std::vector<Canvas *> attachedCanvases;
+
+	void startGrab();
 	void drawv(const Matrix &t, const vertex *v) const;
 
 	static StringMap<TextureType, TYPE_MAX_ENUM>::Entry textureTypeEntries[];