Support custom cursors in WebAssembly

Wasm builds of Qt applications don't support custom cursors. QWasmCursor generates a warning here for Qt::BitmapCursor and Qt::CustomCursor shapes and the wasm cursor is then set to "default".  We're have a QGraphicsScene/QGraphicsView-based application that gets compiled to both a desktop application and a wasm build, and we were hoping to use custom cursors.  Instead, the custom cursors don't work in the wasm build and we get a log file full of warnings.