plugins/screencast: set appropriate minFramerate

PipeWire 1.6.0 seems to have stricter checks on the default vs minimum range. Currently KWin has a lower default than the minimum which breaks negotiation. Fix by setting the minimum to zero.

0/1 should be selectable in order to do variable rate capture (exceeding host screen refresh/letting clients pace), as KWin's pacing currently doesn't work well when set to the host rate (~3fps drop on a 60fps stream).

Signed-off-by: Conn O'Griofa connogriofa@gmail.com