Don't compare viewport and scissor in the pipeline cache key

The viewport and scissor rects were part of the equality operator
for QSSGRhiGraphicsPipelineState and therefore the graphics
pipeline cache key. Both are dynamic state that is set on the
command buffer and never baked into a QRhiGraphicsPipeline, so
comparing them only made the cache key size dependent. Every new
window size created a new set of pipelines for all active shader
and pass combinations, and the old ones were kept in the cache
until the rhi context was torn down.

Pick-to: 6.12 6.11 6.8
Task-number: QTBUG-138669
Change-Id: I56adc2971c7e501fa320e0a17dcead246bd6ed35
Reviewed-by: Andy Nichols <andy.nichols@qt.io>