app_server: plug DrawState instance leak, other minor fixes

* OffscreenCanvas was not deleting its DrawState. Found thanks to
  the allocation tracking feature in libroot_debug (thanks mmlr!)

* Also a missing nothrow and a missing ref release in an error case

src/servers/app/Canvas.cpp

@@ -262,9 +262,10 @@ Canvas::BlendLayer(Layer* layer)
 	fDrawState->SetBlendingMode(B_PIXEL_ALPHA, B_ALPHA_COMPOSITE);
 	fDrawState->SetTransformEnabled(false);
 
-	AlphaMask* mask = new UniformAlphaMask(layer->Opacity());
+	AlphaMask* mask = new(std::nothrow) UniformAlphaMask(layer->Opacity());
 	if (mask == NULL) {
 		layerBitmap->ReleaseReference();
+		layer->ReleaseReference();
 		return;
 	}
 
@@ -299,6 +300,12 @@ OffscreenCanvas::OffscreenCanvas(DrawingEngine* engine,
 }
 
 
+OffscreenCanvas::~OffscreenCanvas()
+{
+	delete fDrawState;
+}
+
+
 void
 OffscreenCanvas::ResyncDrawState()
 {

src/servers/app/Canvas.h

@@ -91,6 +91,7 @@ class OffscreenCanvas : public Canvas {
 public:
 							OffscreenCanvas(DrawingEngine* engine,
 								const DrawState& state, const IntRect& bounds);
+	virtual					~OffscreenCanvas();
 
 	virtual DrawingEngine*	GetDrawingEngine() const { return fDrawingEngine; }