mtmd : add hard limit on image resolution for qwen2vl / qwen2.5vl

[ngxson]

Fixes #13414

Models accepting dynamic resolution (Pixtral / Mistral Small / Qwen VL), we want to:

• Have a max resolution. If image is bigger than the max res, it will be downscaled
• Do a warm up with a more reasonable resolution instead of the max res, otherwise many users will get OOM (tbh I'm not quite sure if this is the best solution, but let's try this and also add a custom max_image_size in the future)
• Resize the image to a multiple of patch_size (or in the case of Qwen VL, must be patch_size * 2)

Btw @ggerganov while working on this, I realized that GGML_PAD only works with multiple power of 2, is this expected?

[ggerganov]

Yes, GGML_PAD works with only powers of 2. Should add a comment to clarify that.