visionOS: Remove the render target size workaround

QRhiShadingRateMap::logicalSize() now exposes the logical size of a
shading rate map, the Metal backend uses it for viewports and scissors
alike, and the Qt Quick scene graph lays out and clips in that space.
Reporting the logical size as the render target size is therefore no
longer needed, and the wrappers for the drawable's color and depth
textures go back to their actual size. The render size, which the window
is sized to, is still taken from the view's viewport.

This partially reverts commit 74c13236be43ee9b0782dba177eb4517dfffb7df,
only the render target size part is undone.

This depends on the qtbase change that adds
QRhiShadingRateMap::logicalSize() and on the corresponding qtdeclarative
change.

Change-Id: I615d6861247b3aacf2883be5a8bb070ea9c05289
Reviewed-by: Andy Nichols <andy.nichols@qt.io>