Update license statements from GPL to zlib, as per the LICENSE file.

This commit updates the license statements in the source files to match LICENSE and src/LICENSE.

Note that I only did this for files that were marked as copyright David White. The eglport files are still marked as GPL because they had a different author.

There were already a few zlib-licensed files with slightly different spacing. I only noticed them partway through. I changed those to have matching whitespace. (The newer spacing fits in an 80×25 vi window ;) )

Thank you for switching to the zlib license.