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This is a convenience to allow renderers to figure out the sample
count for their pso by just querying renderTarget()->sampleCount()
where the render target is set up and provided from another component
outside of the renderer's knowledge and control.

These may be used by Q3DS for HDR images (IBL).

do not reimplement the same logic that is already there in
qopenglfunctions.cpp

...so that setting QRhiSwapChain::sRGB behaves identically between
all 4 backends.

Partial (source size specified) 32-bit uploads no longer read
potentially out of bounds.

8 and 16 bit formats give correct results even when the source
stride is not 4 byte aligned.

The order is important here, if there are multiple operations
targeting or reading from the same texture.

Inspired by QObject::deleteLater()...

...and add a semi-format that can be used to port code with the
'if modernGL then GL_R8 else GL_ALPHA' pattern (in order to support GLES
2.0).

...which comes handy to avoid problems of alloc, destroy, alloc
-> same address as before.

the non-rgba8 formats are a todo for now