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/Graphics.cpp

@@ -156,6 +156,7 @@ StringMap<Graphics::PointStyle, Graphics::POINT_MAX_ENUM> Graphics::pointStyles(
 StringMap<Graphics::Support, Graphics::SUPPORT_MAX_ENUM>::Entry Graphics::supportEntries[] =
 {
 	{ "canvas", Graphics::SUPPORT_CANVAS },
+	{ "hdrcanvas", Graphics::SUPPORT_HDR_CANVAS },
 	{ "pixeleffect", Graphics::SUPPORT_PIXELEFFECT },
 	{ "npot", Graphics::SUPPORT_NPOT },
 	{ "subtractive", Graphics::SUPPORT_SUBTRACTIVE },