As we may not have the doc landing page once integrating into
some other Qt module.

It is not symmetric with beginPass() and endPass() otherwise (as those do
allow null).

As we assume the texture will then be sampled afterwards so a store
action of DontCare is potentially wrong.

Introduce a new getter in QRhi.

Out of my league and time is up. Cannot get it work on non-GL. Something
is wrong with the math, no idea what.

...not yet finished.

Shaders and camera logic not yet there.

It is essential to have a test that exercises depth-only passes and
rendering into a depth texture that is then used in another pass. Some
simple shadow casting directional light is a good example for this.

...when running on gles 3.0+ or gl.

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