Fix incorrect API dec in font class

Forgot to change Integer to Long, lol
If FreeImage is missing, return a blank thumbnail instead of no thumbnail
Abandon layer premultiplication in advance if layer is 24bpp
Move SetDIBitsToDevice dec into public module

It is used a lot of places - no need for them to each have a custom
declare
Overhauled (and much-improved) JPEG export dialog

New features include:

1) Live previews!  I have wanted this *for so long*.  All work is done
in memory, so the hard drive is not touched by the previewer.  Though a
small addition, I consider this one of the best things I have added to
PD in a long time in terms of my personal workflow.

2) Command bar added, so full support for custom presets, auto-save
last-used settings, etc.

3) Necessary interface update to account for the new features.  I'm not
thrilled about the odd aspect size of the new dialog, but the preview is
worthless if it's too small, so this will have to do for now.