[Core] Flycast Redo 1

[JoeMatt]

What does this PR do

• add a libretro fork of dreamcast

Status

• builds, links and runs but no video.
• Logs show that the game is loading, you may even hear audio but no video yet

[github-actions]

Status Update from PR #2373 Analysis

Audited the feature/new-cores/libretrogles branch (PR #2373) vs current develop.

What PR #2373 added to Flycast

The PR introduced a dual-mode approach — wrapping Flycast as a libretro core via #ifdef LIBRETRO guards:

• PVFlycastCore.h: Changed base class from PVLibRetroGLESCore to PVLibRetroGLESCoreBridge, removed upstream Flycast headers (commented out), added extern declarations for kcode, rt, lt, vks, joyx/joyy arrays under #ifdef LIBRETRO
• PVFlycastCore.mm: Removed direct Flycast includes, added #ifdef LIBRETRO init path, added getVariable: for core options (oddly references MelonDS vars — likely copy-paste placeholder)
• PVFlycastEmuCore.swift: Fixed _bridge type from PVFlycastEmuCoreBridge to PVFlycastCoreBridge, added didMoveJoystick forwarding (previously only button press/release were forwarded), commented out keyDown/keyUp
• PVFlycastCore+Controls.mm: Added #ifdef LIBRETRO extern declarations for controller arrays

Status as of PR

• Builds and links
• Game loads, audio may play
• No video output — the video/GL render path for libretro mode was not completed

What's been done on develop since then

• supportsVirtualKeyboard/supportsVirtualMouse API added to PVEmulatorCore
• Full virtual keyboard + mouse overlay system merged
• libretro GL pipeline improvements (DosBox GL fixes, pixel format fixes)
• RA core allow 3d gles cores now — RetroArch wrapper now supports 3D GLES/GL cores
• VecX hardware rendering enabled (was forced to software before)

What still needs doing for native Flycast (non-RetroArch)

1. Video/GL render path: The retro_hw_render_callback setup for OpenGL/Vulkan is not wired to the Metal/MoltenVK layer. This is the same blocker as BeetlePSX ([Core] Beetle PSX #2368) — the hw_render environment callback needs to provide a real OpenGL or Vulkan context.
2. getVariable: placeholders: The core options in PVFlycastCore.mm reference MelonDS variables — needs actual Flycast-specific options.
3. Joystick forwarding from PR is good and should be preserved when this is revived.

Path forward

Option A: Via RetroArch wrapper — Flycast is already available as a RetroArch core with full GL support. The RA core allow 3d gles cores now change on develop means this should work via the RA path without a native core.

Option B: Native libretro mode — Requires completing the retro_hw_render_callback → Metal/MoltenVK bridge, similar to what DuckStation has done.

Related: #2373 (parent PR), #2482 (Core Updates epic)