Add reliable back-buffer movie capture

[caseychaos1212]

Summary

This PR adds a reliable, explicitly driven back-buffer movie-capture path for tools such as the Qt W3D Viewer.

The new API captures rendered Direct3D back-buffer frames into AVI output while reporting initialization and frame-capture failures to the caller.

Changes

• Add WW3D::Try_Start_Movie_Capture_From_Back_Buffer().
• Add WW3D::Try_Update_Movie_Capture_From_Back_Buffer().
• Track whether capture is using the legacy front buffer or the new back buffer.
• Prevent back-buffer capture from also being advanced automatically by Begin_Render().
• Copy back-buffer data through a lockable system-memory staging surface.
• Convert pitched BGRA32 input into bottom-up, DWORD-aligned BGR24 AVI frames.
• Validate filenames, dimensions, frame rates, formats, strides, and allocation sizes.
• Stop capture and release resources cleanly when conversion, locking, or AVI writing fails.
• Harden the existing front-buffer capture path with the same conversion and failure handling.
• Preserve numbered AVI filename generation.

Compatibility

The existing movie-capture and FrameGrabClass entry points remain available. The new methods provide an isolated, Boolean-returning API for callers that need explicit back-buffer capture.

Back-buffer capture remains Windows-specific and rejects unsupported or multisampled surface formats.

Testing

New tests cover:

• AVI row-stride calculation and overflow handling.
• Pitched BGRA32-to-BGR24 conversion.
• Vertical row flipping and AVI row padding.
• Source and destination boundary protection.
• Invalid pointers, dimensions, pitches, and strides.
• Invalid or oversized FrameGrabClass construction.
• Existing and new API signatures.

MSVC x64 builds passed with both frame-grab and screenshot tests passing. Strict MSVC x86 /W4 /WX builds also passed with 2/2 tests passing.