plugins/screencast: Hard-code the maximum cursor size to (256, 256)

XCURSOR_SIZE * scale factor is not the way to compute the current cursor
size. For example, with breeze cursor theme at an output with a scale of
2 and cursor size 24, cursor images will have the effective size of (64, 64).

Also, the cursor can change when passing over user interface elements.

In order to accommodate for all of that, this change makes kwin reserve
enough of space for a cursor of size 256x256. "256" is a magical number
that comes from DRM. With many drivers, the maximum cursor size is 256.

BUG: 448840


(cherry picked from commit d2867fc4)