Make (and verify) public/ files compile under C.

This involves adding some missing extern "C" { } declarations,
using FPDF_ types instead of C++ types, and converting pass
by reference arguments into pointers.

Test this using fpdfview_embedertest for simplicity.

BUG=pdfium:158
R=thestig@chromium.org

Review URL: https://codereview.chromium.org/1130843003
14 files changed