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/wrap_Canvas.h

@@ -43,6 +43,7 @@ int w_Canvas_getWrap(lua_State *L);
 int w_Canvas_clear(lua_State *L);
 int w_Canvas_getWidth(lua_State *L);
 int w_Canvas_getHeight(lua_State *L);
+int w_Canvas_getType(lua_State *L);
 extern "C" int luaopen_canvas(lua_State *L);
 
 } // opengl