ScreenshotEffect: add format parameter to some screenshot function

Adds a format option, allowing to use in-plus of the default current PNG format, a raw Image encoding.

BMP encoding was tested but not as fast.

Small benchmark:

On my systems with 1920x1080+2560x1440 setup, with a patched spectacle measuring duration between before the dbus call to the end of the image decoding.

Before:

Fullscreen screenshot: 644ms to 1000ms

Rectangular selection: 1000ms to a lot more unreliable

After:

Fullscreen screenshot: 220 to 359ms

Rectangular selection: 490ms to 530ms