Add imlib2 support for login screen customisation

[matt335672]

Fixes #1929 #1819

See also discussion #1931

This PR allows for more customization of the xrdp login screen. An example customised screen from an Ubuntu 20.04 VM is:-

This is achieved by adding the following parameters to xrdp.ini:-

ls_background_image=/usr/share/backgrounds/ryan-stone-skykomish-river.jpg
ls_background_transform=zoom

ls_logo_filename=/usr/share/icons/hicolor/256x256/apps/ubuntu-logo-icon.png
ls_logo_transform=zoom
ls_logo_width=140
ls_logo_height=140
ls_logo_x_pos=105
ls_logo_y_pos=50

File types other than bmp files are supported by way of the imlib2 library which is available on all our currently supported platforms. Imlib2 support is optional. Without it, we fall back to the existing bmp only code. The existing code has been reworked to (hopefully) make it simpler to understand, and also to fix #1819.

Alpha blending of images onto the background colour behind them is supported - this can be seen on the Ubuntu logo in the screenshot above. On multi monitor setups, the background image is displayed on the primary monitor only.

A test suite has been added for the extended xrdp_bitmap_load() call, so that the correct operation of the external library can be checked.

This is a fairly significant PR, and so I suggest we hold off merging it until after the next release.

[matt335672]

Just re-read #1931. I've added an extra check so that wallpaper isn't loaded if the client has disabled wallpaper in the performance settings.

[metalefty]

I've added an extra check so that wallpaper isn't loaded if the client has disabled wallpaper in the performance settings.

Awesome.

[metalefty]

Overall, LGTM.

[metalefty]

Auto-detection is generally very nice but autodetected libraries will not appear in xrdp -v. I want an easy way to tell users if your compiled xrdp supports png/jpg images on the login screen. ldd /usr/sbin/xrdp sounds a little bit guru's way.

If we introduce more auto-detection, we might need to invent a new way to show which libraries/features compiled with.

[matt335672]

That's a really good point - I hadn't thought of that but it's a feature I use a lot when triaging problems. I'll remove the auto-detect.

[matt335672]

xrdp -v now works OK for imlib2.

I also noticed I'd not added CI support for i386 legacy. It's a small thing, but it's in now.

[metalefty]

so I suggest we hold off merging it until after the next release.

I agree.