Fix PixtralProcessor patch_size when spatial_merge_size is used
Author: mgoin
Pull request: yes

# What does this PR do?

Fixes an image resizing issue for Mistral3 that I found when integrating into vLLM (https://github.com/vllm-project/vllm/pull/15505)

The issue was found when processing [this image](https://careforplant.com/wp-content/uploads/2024/01/1677123490_gagaru-club-p-krasivie-malenkie-tsvetochki-krasivo-52-min-1-1080x675.jpg). The mistral processor was producing 975 image tokens and the hf processor was producing 936 tokens, causing a mismatch in the number of expected placeholder tokens in the input_ids.

As reference in mistral-common, I see that they always make sure to take `spatial_merge_size` into account when resizing the image.
https://github.com/mistralai/mistral-common/blob/6e637437fe4795353b7ba19cc8479c124b95b580/src/mistral_common/tokens/tokenizers/multimodal.py#L143-L144
and
https://github.com/mistralai/mistral-common/blob/6e637437fe4795353b7ba19cc8479c124b95b580/src/mistral_common/tokens/tokenizers/multimodal.py#L164-L167

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