Implement FlipRotate without WIC for non-Windows platforms

[Joel Kiptoo (Kiptoo-Deus)]

Fixes #350.

FlipRotate was the only image operation that always used WIC, so DirectXTexFlipRotate.cpp was only built on Windows and the functions were declared under #ifdef _WIN32.

This builds it on all platforms. On Windows nothing changes: the WIC path and the float16/float32 conversion path are the same code, now inside #ifdef _WIN32. On other platforms, a new PerformFlipRotate moves whole pixels directly:

• Supported: formats that are not compressed, packed, planar or palettized and have a whole number of bytes per pixel (8 to 128 bits). Other formats return HRESULT_E_NOT_SUPPORTED, checked before the result is allocated.
• Rows are copied with a single memcpy when there's no horizontal flip and no 90/270 rotation.
• Combining a rotation with a flip: the WIC documentation doesn't say which is applied first. I followed Wine's IWICBitmapFlipRotator (dlls/windowscodecs/fliprotate.c), where the flip is applied to the source before the rotation (e.g. TEX_FR_ROTATE90 | TEX_FR_FLIP_HORIZONTAL is an anti-transpose). I couldn't check this against WIC itself, so it would be worth confirming on Windows. It only matters for a 90/270 rotation combined with exactly one flip.

Testing on macOS (arm64, Apple clang), building DirectXTex with its own CMake against the vcpkg directx-headers and directxmath ports:

• The library builds with DirectXTexFlipRotate.cpp included, with no new warnings.
• A test compares both FlipRotate overloads against a straightforward reference (flip the source, then rotate clockwise in 90 degree steps) for every rotation/flip combination, on a 7x5 image with 3 array slices, in R8_UNORM, R16_UNORM, R8G8B8A8_UNORM, R16G16B16A16_FLOAT, R32G32B32_FLOAT and R32G32B32A32_UINT (1 to 16 bytes per pixel). It also checks that BC1_UNORM, NV12, YUY2, R1_UNORM, P8 and R8G8_B8G8_UNORM return HRESULT_E_NOT_SUPPORTED, and that null pixels return E_POINTER. 187 checks pass.
• Making the reference rotate first and flip second fails exactly the 48 cases combining a 90/270 rotation with a single flip, so the test does pin down the order.

I haven't built the Windows configurations. The only change there is the added #ifdef _WIN32 guards around the existing code.

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