wayland: Fix loading of HiDPI cursors

Xcursors don't support hidpi so if a hidpi cursor is needed, kwin will
scale the desired size by the scale factor and ask Xcursor helpers to
load a theme with the given name and the size.

However, the theme loading code doesn't take into account that Xcursor
theme loading helpers may not return cursor sprites of size size * scale
if the theme has no such a size.

For example, if the cursor theme only provides 24, 36, and 48 sizes and
kwin attempts to load cursors of size 48 with a scale factor of 2, we
will get cursors of size 48 instead of 96. Unfortunately, this will
result in the issue where the cursor shrinks when hovering decorations
because kwin doesn't know that the effective scale factor (1) is
different from the requested scale factor (2).

In order to fix loading of HiDPI cursors, we need to approximate the
effective scale factor of every cursor sprite as we load it.