Add support for right-clicking on images (bug 1012805)

[nicolo-ribaudo]

Add support for right-clicking on images

This patch adds right-click support for images in the PDF, allowing
users to download them. To minimize memory consumption, we:

• Do not store the images separately, and instead crop them out of the
  PDF page canvas
• Only extract the images when needed (i.e. when the user right-clicks
  on them), rather than eagerly having all of them available.

To do so, we layer one empty 0x0 canvas per image, stretched to cover
the whole image, and only populate its contents on right click.
These images need to be inside the text layer: they cannot be behind
it, otherwise they would be covered by the text layer's container and
not be clickable, and they cannot be in front of it, otherwise they
would make the text spans unselectable.

This feature is managed by a new preference, imagesRightClickMinSize:

• when it's set to -1, right-click support is disabled
• when set to 0, all images are available for right click
• when set to a positive integer, only images whose width and height are
  greater than or equal to that value (in the PDF page frame of
  reference) are available for right click.

Fixes https://bugzilla.mozilla.org/show_bug.cgi?id=1012805

TODO:

• Fix handling of images that are rotated (see TODO in the code)
• Add tests