fix broken rendering of transparent images when converting to PDF

The fixes for #1512 and #1638 were band-aids but did not fix the
underlying problem. Updating the patched Qt should fix #2214.

CHANGELOG.md

@@ -10,6 +10,7 @@ v0.12.3 (unreleased)
 * **#2183**: apply --javascript-delay settings for non-main resource loaders
 * **#2190**: do not depend on ICU even if it is already installed
 * **#2194**: **[qt]** fix debugging of Qt/WebKit due to missing debug information
+* **#2214**: **[qt]** fix broken rendering of transparent images when converting to PDF
 * **#2280**: do not allow data URIs for --header-html or --footer-html
 * **#2322**: fix broken debug builds with MSVC
 * **#2355**: add support for proxy bypass for specific hosts with --bypass-proxy-for

qt

@@ -1 +1 @@
-Subproject commit 4be9ebb8f8f9af2f24584cebb6a2b44a22545b91
+Subproject commit 7e48a1fac7e0f9aefccd01e9871f987da3a62fda