Fixed ambient layers "keepratio" option

I had broken this in 264be2108c51837fa92085f6c839e66aebbcfc9e by no
longer reading out the property.

Made some further simplifications and two more fixes:

* When the original image's size was not 800x600, it wouldn't get scaled
  as expected due to the integer division being performed before the
  multiplication.

* I had changed the default resolution to 1280x720, while mKeepRatio
  expects to scale the original image as if it was displayed on an
  800x600 screen. So now it's hardcoded on 800x600 rather than using
  the defaultScreenWidth/Height variables.

* Removed the confusing x/y parameters from AmbientLayer::draw, which
  were actually set to the graphics width/height.

* Changed SDLGraphics::drawRescaledImagePattern to not repeatedly
  set the source origin.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/resources/resource.h b/src/resources/resource.h
index 7f1f4836..9fe00f3d 100644
--- a/src/resources/resource.h
+++ b/src/resources/resource.h
@@ -59,7 +59,7 @@ class Resource
         { return mIdPath; }
 
     protected:
-        virtual ~Resource() {}
+        virtual ~Resource() = default;
 
     private:
         std::string mIdPath;    /**< Path identifying this resource. */