plugins/shakecursor: Make the cursor big enough upon the first shake

The current behavior is that the cursor size follows how hard or fast the cursor is shaken. While this looks fancy, given the purpose of this plugin, it should be possible to magnify the cursor as easy as possible without interfering or false triggering.

This change implements a sort of a compromise. When the cursor is shaken for the first time, the cursor is magnified by some certain scale factor. Then every next shake magnifies the cursor further by a smaller amount.