Require stencil=true to be specified in setCanvas if stenciling is used while a Canvas is active and there is no custom stencil buffer.

--HG--
branch : minor

src/common/pixelformat.h

@@ -124,6 +124,11 @@ bool isPixelFormatDepthStencil(PixelFormat format);
  **/
 bool isPixelFormatDepth(PixelFormat format);
 
+/**
+ * Gets whether the specified pixel format is a stencil type.
+ **/
+bool isPixelFormatStencil(PixelFormat format);
+
 /**
  * Gets the size in bytes of the specified pixel format.
  * NOTE: Currently returns 0 for compressed formats.