liquid_fb: Improve resize handling

This patch make the handling of resizing the virtual framebuffer more
consistent. Liquid_fb keeps track of two sizes. The "next size" is the
size of the framebuffer handed out via the next call of 'dataspace'.
The "designated size" is the size as demanded by the user. The latter
size may be updated more often than the "next" size, depending on the
responsiveness of the framebuffer client to mode-change signals.

The patch also removes the synchronization with refresh calls because
the synchronization made the flickering artifacts worse when executing
nitpicker within liquid_fb. So it was not properly working anyway.
In the future, we might reimplement such a synchronization mechanism
when switching to the server API.

Issue #1056
nitpicker: Support for dynamic screen resizing

Fixes #1056