[XembedSNIProxy] If available, always use 32-bit color.

Summary:
If available, always use 32-bit color. We don't need Composite extension to handle transparency - all client windows are our children and transparency in handled in QML, inside the panel (so no need to composite real windows).

BUG: 356937
FIXED-IN: 5.17.1

Test Plan:
Disable compositor, run:
  - pidgin (make sure pidgin-libnotify is NOT installed)
  - keepassx
  - xchat
  - liferea

Before changes: black backgroud
After changes: transparent background

Reviewers: davidedmundson, #plasma_workspaces, #plasma

Reviewed By: davidedmundson, #plasma_workspaces, #plasma

Subscribers: mlaurent, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D24530