Investigate if number of allocations performed while filling up a QRhiResourceUpdateBatch could be reduced

https://codereview.qt-project.org/c/qt/qtbase/+/275432 shows an approach to remove the individual QByteArray data; members from individual DynamicBufferUpdate items inside a resource update batch. Rather, a single, batch-level QByteArray is used there, with the individual update descriptions only storing on offset inside the "pool".

This still allocates/resizes of course, but the there is fewer allocations overall.

Or figure out something else.

It could be that the improvements may (also) come from changes on the client side (i.e. the Qt Quick renderer) instead of or in addition to the QRhi API.