iOS: support filling in of text form fields
iOS: keep reference to the fz_pixmaps.
iOS: use partial updates to display annotation changes
iOS: add support for selecting form choice fields
Fix null device clip stack scissor handling.

Was failing to correctly initialise scissors when extending the stack.
This caused the fill text to disappear in fts_15_109.pdf
page->contents should be a reference, not the original object.
Adopt backported bugfix from openjpeg2 into our version.

If header field is too large, warn, but don't bale out.
Fix cross document store object hash collisions.

When putting store objects into the store, ensure that they do cannot
collide across documents.
Remove duplicated line.

Thanks to Micha for spotting this.
Fix so that multi-threaded rendering of display list works.

Proper use of mutex lock in areas where we multiple threads can
not be accessing the document or page level objects at the same
time.   With this fix, multiple threads can be rendering different
display lists at the same time.

Also fix for use of page selection scroll bar so that it does not
render the page until the user lifts off the bar.  This is the
same as the Android app and avoids smaller devices getting overloaded
if you do rapid scrolling in a document.