feat: Add NVIDIA GPU process utilization and memory

[KinanLak]

This PR adds NVIDIA GPU support to the process list view :

• adds per-process GPU metrics (utilization + memory), using NVML when available, with /proc/[pid]/fdinfo fallback
• adds new process sorting keys: gpu and gpu memory
• adds a proc_gpu_only filter (toggle g) to show only GPU-active processes
• adds a proc_gpu_graphs option to display per-process GPU graphs
• wires the feature end-to-end across config, menu, input, collector, and rendering.

Also fix : #968

[KinanLak]

Ready to review / merge

[Copilot]

Pull request overview

This PR adds comprehensive per-process GPU monitoring to btop, implementing the feature requested in issue #968. The implementation provides GPU utilization and memory usage metrics for individual processes, supporting both NVIDIA GPUs (via NVML) and other GPU vendors (via /proc/[pid]/fdinfo parsing).

Changes:

• Adds per-process GPU metrics (utilization % and memory) with dual-source collection: NVML for NVIDIA GPUs and fdinfo parsing as fallback
• Introduces new sorting options ("gpu" and "gpu memory") and a GPU-only filter toggle (key 'g') to display only processes with active GPU usage
• Implements optional per-process GPU graphs alongside existing CPU graphs with responsive column layouts

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/linux/intel_gpu_top/CMakeLists.txt	Adds _GNU_SOURCE definition to fix implicit declaration of asprintf
src/linux/btop_collect.cpp	Core implementation: adds helper functions, NVML process stats collection, fdinfo parsing for non-NVIDIA GPUs, and GPU-only filtering logic
src/btop_shared.hpp	Extends proc_info struct with gpu_p, gpu_m, and gpu_t fields; adds "gpu" and "gpu memory" to sort_vector
src/btop_shared.cpp	Updates sorting logic to handle new GPU metrics (indices 6-7) and shifts CPU metrics to indices 8-9; adds GPU filter check in matches_filter
src/btop_menu.cpp	Adds help text for new config options (proc_gpu_graphs, proc_gpu_only) and updates proc_sorting description
src/btop_input.cpp	Adds 'g' key binding to toggle GPU-only filter
src/btop_draw.cpp	Implements GPU column rendering with width-responsive layouts, GPU graphs, and gpu-only toggle indicator
src/btop_config.cpp	Adds configuration options for proc_gpu_graphs and proc_gpu_only

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