Use system openjpeg2 library

Upstream added the ability to use the system openjpeg2 library instead of an embedded version, but it is disabled by default.

This commit includes a patch to edit the pdfium.gni file to enable the desired option. I must admit that I am not very familiar with the .gni architecture, so someone might know of a way to use a build flag to achieve the same results without the need for a patch.