[wayland] Allow nested mode to have a fractional scale < 1

The bug reporter presents a use case of wanting to run a game and scale
down the result.

For everyone else kwin_wayland nested is just a test tool, so I don't
see any disadvantages.

It could be useful in testing.

BUG: 422141

main_wayland.cpp

@@ -609,7 +609,7 @@ int main(int argc, char * argv[])
             return 1;
         }
         const qreal scale = parser.value(scaleOption).toDouble(&ok);
-        if (!ok || scale < 1) {
+        if (!ok || scale <= 0) {
             std::cerr << "FATAL ERROR incorrect value for scale" << std::endl;
             return 1;
         }