Previously, Imagick uploads of PDF files with non-opaque alpha channels would result in a black background replacing alpha in the generated thumbnail. This patch adds a `remove_pdf_alpha_channel()` function in the Imagick classes to use a white background instead.
Props gitlost, joemcgill, joedolson, launchinteractive, emirpprime, mwtsn, ceer, maysi, madejackson, 6adminit, costdev, oglekler.
Fixes#39216.
git-svn-id: https://develop.svn.wordpress.org/trunk@56271 602fd350-edb4-49c9-b593-d223f7449a82
