Merge tag '0.3.1' into develop

Version 0.3.1
Pass event to sc_mouse_zoom to prevent SIGSEGV
Additional check for event null pointer in sc_zoom
Fix documentation

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>
Only enable reparent if we have X11

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>
Do not refresh if the zoom level does not change
Cache thumbnails to support smooth zooming
Add Lingzhu Xiang to AUTHORS
CS

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>
Correct the given scale value
Move input validation back to document_open

Re-setting scale back to 1 does not make sense if the change to scale comes from
zooming in zathura.

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>
Do not adjust scale if given scale is not positive

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>
CS

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>
Check memory allocation
Avoid large amount of scaling for pages at low zoom level
Penalize large render jobs during zooming

All but the last jobs during zooming are aborted, so let smaller
jobs go faster.
Make thumbnail maximum size configurable
CS

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>
Merge remote-tracking branch 'xlz/develop' into develop
Fix typo in synopsis in man page.