Fix #187: Use A8R8G8B8 fallback for radar overlay and shroud formats

[fbraz3]

Description

Fixes #187.

On macOS (and potentially Linux) in fullscreen mode, the presentation format is negotiated as D3D9Format::X8R8G8B8 (without an alpha channel).
When the game checks for texture support via CheckDeviceFormat using the backbuffer format, DXVK/MoltenVK returns that A8R8G8B8 and A4R4G4B4 are unsupported relative to X8R8G8B8.
This causes W3DRadar::initializeTextureFormats() to fall back to WW3D_FORMAT_X8R8G8B8 (the only fallback in the original implementation of findFormat).
Because X8R8G8B8 does not have an alpha channel, the overlay and shroud (fog of war) textures render as solid opaque screens, causing the minimap to appear completely black/dark.

Changes

• Updated findFormat in W3DRadar.cpp to accept a defaultFallback parameter (defaulting to WW3D_FORMAT_X8R8G8B8).
• Changed the fallback for the radar overlay and shroud textures to WW3D_FORMAT_A8R8G8B8. Modern Vulkan devices support A8R8G8B8 textures natively even if DXVK's capability checks return false under X8R8G8B8 swapchains.