About the unit test sample files.

These files are added in here to support unit testing of Poppler
functionality. 

orientation.pdf: a file containing one page in each major
orientation. The first page is portrait, the next is landscape
(portrait rotated 90 degrees clockwise), the next is upside down
(portrait rotated 180 degrees), and the final is seascape (portrait
rotated 90 degrees counterclockwise).
This file has permissions set, such that only low resolution printing
is allowed, inserting, deleting and rotating pages is allowed, copying
content is not allowed, and text reading is allowed for accessibility.
The password is "foo".

FullScreen.pdf, UseAttachments.pdf, UseNone.pdf, UseOC.pdf,
UseThumbs.pdf: simple files demonstrating various PageMode 
options (where the name of the file matches the PageMode option).

PasswordEncrypted.pdf: single blank page, created with 
Acrobat 7. This is encrypted with AES128. The password is "password".

WithAttachments.pdf: a simple file with two files attached/ embedded. 

stroke-alpha-pattern.pdf:  Taken from https://gitlab.freedesktop.org/poppler/poppler/-/issues/178
This file contains a single page with different uses of axial shadings
with different opacity settings.  In particular it contains a fill
with an axial shading that is clipped to a path.  In this case,
the stroke opacity has to be used rather than the fill opacity.
