mtmd: correct get_n_pos / get_decoder_pos

[ngxson]

Overview

Continue #22161

• Add a new enum mtmd_pos_type for using internally (the idea is to support falcon-ocr positioning style in near future)
• Also update get_n_pos and get_decoder_pos to return the correct positioning style

Test results:

[vision] OK:   ggml-org/SmolVLM-500M-Instruct-GGUF:Q8_0
[vision] OK:   ggml-org/SmolVLM2-2.2B-Instruct-GGUF:Q4_K_M
[vision] OK:   ggml-org/SmolVLM2-500M-Video-Instruct-GGUF:Q8_0
[vision] OK:   ggml-org/gemma-3-4b-it-GGUF:Q4_K_M
[vision] OK:   THUDM/glm-edge-v-5b-gguf:Q4_K_M
[vision] OK:   second-state/Llava-v1.5-7B-GGUF:Q2_K
[vision] OK:   cjpais/llava-1.6-mistral-7b-gguf:Q3_K_M
[vision] OK:   ibm-research/granite-vision-3.2-2b-GGUF:Q4_K_M
[vision] OK:   second-state/MiniCPM-Llama3-V-2_5-GGUF:Q2_K
[vision] OK:   openbmb/MiniCPM-V-2_6-gguf:Q2_K
[vision] OK:   openbmb/MiniCPM-o-2_6-gguf:Q4_0
[vision] OK:   bartowski/Qwen2-VL-2B-Instruct-GGUF:Q4_K_M
[vision] OK:   ggml-org/Qwen2.5-VL-3B-Instruct-GGUF:Q4_K_M
[vision] OK:   ggml-org/InternVL2_5-1B-GGUF:Q8_0
[vision] OK:   ggml-org/InternVL3-1B-Instruct-GGUF:Q8_0
[vision] OK:   ggml-org/Qwen2.5-Omni-3B-GGUF:Q4_K_M
[vision] OK:   ggml-org/LFM2-VL-450M-GGUF:Q8_0
[vision] OK:   ggml-org/granite-docling-258M-GGUF:Q8_0
[vision] OK:   ggml-org/LightOnOCR-1B-1025-GGUF:Q8_0
[vision] OK:   ggml-org/DeepSeek-OCR-GGUF:Q8_0
[vision] OK:   ggml-org/dots.ocr-GGUF:Q8_0
[vision] OK:   ggml-org/HunyuanOCR-GGUF:Q8_0
[vision] OK:   ggml-org/gemma-4-E2B-it-GGUF:Q8_0
[audio]  OK:   ggml-org/ultravox-v0_5-llama-3_2-1b-GGUF:Q8_0
[audio]  OK:   ggml-org/Qwen2.5-Omni-3B-GGUF:Q4_K_M
[audio]  OK:   ggml-org/Voxtral-Mini-3B-2507-GGUF:Q4_K_M
[audio]  OK:   ggml-org/LFM2-Audio-1.5B-GGUF:Q8_0
[audio]  OK:   ggml-org/gemma-4-E2B-it-GGUF:Q8_0
[audio]  OK:   ggml-org/Qwen3-ASR-0.6B-GGUF:Q8_0
[vision] OK:   ggml-org/pixtral-12b-GGUF:Q4_K_M
[vision] OK:   ggml-org/Mistral-Small-3.1-24B-Instruct-2503-GGUF
[vision] OK:   ggml-org/Qwen2-VL-2B-Instruct-GGUF:Q4_K_M
[vision] OK:   ggml-org/Qwen2-VL-7B-Instruct-GGUF:Q4_K_M
[vision] OK:   ggml-org/Qwen2.5-VL-3B-Instruct-GGUF:Q4_K_M
[vision] OK:   ggml-org/Qwen2.5-VL-7B-Instruct-GGUF:Q4_K_M
[vision] OK:   ggml-org/Qwen3-VL-2B-Instruct-GGUF:Q8_0
[vision] OK:   ggml-org/InternVL3-8B-Instruct-GGUF:Q4_K_M
[vision] OK:   ggml-org/InternVL3-14B-Instruct-GGUF:Q4_K_M
[vision] OK:   ggml-org/Qwen2.5-Omni-7B-GGUF:Q4_K_M
[vision] OK:   ggml-org/GLM-4.6V-Flash-GGUF:Q4_K_M
[audio]  OK:   ggml-org/ultravox-v0_5-llama-3_1-8b-GGUF:Q4_K_M
[audio]  OK:   ggml-org/Qwen2.5-Omni-7B-GGUF:Q4_K_M

Requirements

• I have read and agree with the contributing guidelines
• AI usage disclosure: no