SurfaceInterface::scaleOverride() doesn't scale the bufferScale, so if the
scale override is 2, but the buffer scale specified by the client is 1,
bufferScale() will return 1.
Xwayland cursor surface implicitly relied on this behavior.
Porting cursor to SurfaceItem changed that. Now cursor surfaces honor
the scale override, which makes Xwayland cursors too small.
In order to properly fix, plasma has to scale Xcursor.size in xrdb. The
problem is that plasma also sets XCURSOR_THEME and XCURSOR_SIZE envvars
that take precedence. Plasma must stop setting those envvars, but it's
doable only with Qt 6.5, which got MouseCursorTheme and MouseCursorSize
hints in QPlatformTheme.
The cursor scene contains the contents of the cursor. It contains a
CursorItem. The CursorItem either creates a SurfaceItem or an ImageItem
based on the currently attached CursorSource.
The cursor item is rendered by the cursor scene. For now, wherever the
cursor must be rendered, a dummy scene delegate is constructed. It's not
nice but it's a pretty cheap operation. There's a lot of potential for
clean up by moving cursor layer handling from output backends to
compositor. The main reason why there are no persistent scene views is
that it's just easier than tracking when they are actually used, e.g.
after switching between hw and sw cursor.
The software cursor fallback is a bit tricky case. It made to work by
constructing a scratch fbo. The cursor scene is rendered in the scratch
fbo, which is then rendered on the screen. Similar to the case above,
there's space for improvements, but I don't think it has to block the
effort for reusing Items to render the cursor.
