Use next's GPU metrics implementation for GPU_fdinfo class

[17314642]

Every driver except for amdgpu and nvidia will now use gpu metrics from mangohud-next

I tested that it works on Intel ARC A770 using both 64-bit and 32-bit builds and mangoapp in gamescope.
proc_vram using fdinfo from mangohud-next for amdgpu also works

I implemented it using opaque pointer technique to not expose any headers from next to legacy and also making as little changes as possible to the mangohud-next's codebase.

[flightlessmango]

Could you squash or reorder e16441d and a5b77df so the amdgpu fdinfo path is not broken on bisect?

../../src/gpu_fdinfo.cpp:23:15: error: base operand of ‘->’ has non-pointer type ‘LegacyGPUWrapper’
   23 |     return gpu->get_process_vram_used(pid);
      |               ^~
ninja: build stopped: subcommand failed.```