Fix Issue #476: Add HDR Canvas (thanks, Alexander Szpakowski!).

* Add `love.graphics.isSupported("hdrcanvas")' to query whether the hardware
  actually supports HDR canvases.

* Add optional third argument to `love.graphics.newCanvas(w,h, type)', where
  `type' is one of "normal" or "hdr" and defaults to "normal". HDR canvases
  use floating point that can have values > 1 to store pixels, allowing for
  HDR rendering and other higher level lighting techniques when combined with
  pixel effects.

* Add `canvas:getType()'.

src/modules/graphics/opengl/Graphics.h

@@ -269,7 +269,7 @@ public:
 
 	ParticleSystem *newParticleSystem(Image *image, int size);
 
-	Canvas *newCanvas(int width, int height);
+	Canvas *newCanvas(int width, int height, Canvas::TextureType texture_type = Canvas::TYPE_NORMAL);
 
 	PixelEffect *newPixelEffect(const std::string &code);