Fixes for re-enabling more MSVC level 4 warnings: pdfium/ edition (try 2)

This contains fixes for the following sorts of issues:
* Nonstandard extension: unnamed struct/union

This also changes the signature of FPDF_FillRect() to enable the cleanest
possible changes in https://codereview.chromium.org/372273005 .

BUG=81439
TEST=none

Original patch by Peter Kasting <pkasting@chromium.org>
Original code review: https://codereview.chromium.org/376003003/

R=pkasting@chromium.org

Review URL: https://codereview.chromium.org/377353002