wasm Canvas QML render incorrectly on highdpi screens

In wasm, QML Canvas rendered to QImage on highdpi screen renders with wrong size.

In `QSGOpenGLAtlasTexture::Atlas::upload`, a tmp QImage is created and
filled with the rendered QImage that had a devicePixelRatio set to screen dpr.
This cause calls `drawImage` to effectivly apply devicePixelRatio a second time.