Reprocess .png's as transparent (w/ GraphicsMagic since my local

copy of ImageMagic creates garbage transparent .png's).

PR: 30190
Backport: r570373


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@570374 13f79535-47bb-0310-9956-ffa450edef68