Fix staging buffer required size calculation (fixes#1056)
The `required_staging_buffer_size` is currently calculated differently in two places, each will be correct in different situations:
* `prepare_staging_buffers()` based on actual `buffer_byte_len()`
* `set_required_staging_buffer_size_to_max()` based on item_size
In the case of render assets, `prepare_staging_buffers()` would only operate over changed assets. If some of the assets didn't change, their size wouldn't be taken into account for the `required_staging_buffer_size`. In some cases, this meant the buffers wouldn't be resized when they should. Now `prepare_staging_buffers()` is called over all assets, which may hit performance but at least gets the size right.
Shortly after `prepare_staging_buffers()`, `set_required_staging_buffer_size_to_max()` would unconditionally overwrite the previously computed value, even if using `item_size` made no sense. Now it only overwrites the value if bigger.
This can be considered a short term hack, but should prevent a few hard to debug panics.
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