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.cpp

@@ -181,6 +181,16 @@ int w_Canvas_getHeight(lua_State *L)
 	return 1;
 }
 
+int w_Canvas_getType(lua_State *L)
+{
+	Canvas *canvas = luax_checkcanvas(L, 1);
+	Canvas::TextureType type = canvas->getTextureType();
+	const char *str;
+	Canvas::getConstant(type, str);
+	lua_pushstring(L, str);
+	return 1;
+}
+
 static const luaL_Reg functions[] =
 {
 	{ "renderTo", w_Canvas_renderTo },
@@ -192,6 +202,7 @@ static const luaL_Reg functions[] =
 	{ "clear", w_Canvas_clear },
 	{ "getWidth", w_Canvas_getWidth },
 	{ "getHeight", w_Canvas_getHeight },
+	{ "getType", w_Canvas_getType },
 	{ 0, 0 }
 };