Review Request 128763: WindowThumbnail: Do GL calls in the correct thread

[David Edmundson]

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https://git.reviewboard.kde.org/r/128763/
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Review request for KDE Frameworks and Plasma.


Repository: plasma-framework


Description
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WindowThumbnail did some open GL operations, discarding old textures, in the GUI thread. Whislt it's not going to cause a threading issue (as updatePaintNode always ran when the main thread was blocked) we're not meant to mix threads with openGL contexts.

It also seems to have a GL leak on nvidia, which was previously masked
by the double delete fixed in https://git.reviewboard.kde.org/r/126131/diff/2/
It seems only one worked, and in the applied version we went with the wrong one.

This patch makes use of QQuickItem::releaseResources to delete the GL
textures on window change and destructor; it's then removed from
stopRedirecting so that start/stop redirecting handles xcb on the GUI thread and updatePaintNode/discardPixmap is the GL stuff on the render thread. 

See http://doc.qt.io/qt-5/qquickitem.html#graphics-resource-handling

REVIEW:


Diffs
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  src/declarativeimports/core/windowthumbnail.h 7276f95de16e71006618f3282d8eaf419a199d1d 
  src/declarativeimports/core/windowthumbnail.cpp d106994315099ab6e6f948c31a606d5309ae03e2 

Diff: https://git.reviewboard.kde.org/r/128763/diff/


Testing
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Using nvidia with proprietory drivers (which puts me QSG_RENDER_LOOP=threaded) mouse over the panel a lot. VRAM didn't increase. Previews still appear. 
"Used Dedicated Memory:" in nvidia-settings remained roughly static, rather than constantly increasing.


Thanks,

David Edmundson

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