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/Graphics.cpp

@@ -158,6 +158,7 @@ StringMap<Graphics::Support, Graphics::SUPPORT_MAX_ENUM>::Entry Graphics::suppor
 {
 	{ "canvas", Graphics::SUPPORT_CANVAS },
 	{ "hdrcanvas", Graphics::SUPPORT_HDR_CANVAS },
+	{ "mrtcanvas", Graphics::SUPPORT_MRT_CANVAS },
 	{ "shader", Graphics::SUPPORT_SHADER },
 	{ "npot", Graphics::SUPPORT_NPOT },
 	{ "subtractive", Graphics::SUPPORT_SUBTRACTIVE },