Default to YUV_400 format for .raw extension files (RAW Bayer / CFA patterns)

[for13to1]

Describe the bug

As the author who originally introduced the RAW file support feature to YUView, I wanted to fix a minor logical fallback discrepancy regarding .raw files.
In YUView, .raw extension files (defined in RAW_BAYER_EXTENSIONS) are treated as raw Bayer/CFA sensor data. Since YUView uses YUV_400 (luma-only / single Y plane) internally to represent single-channel raw files, these files should default to the YUV_400 format.

However, if a .raw file is opened without resolution indicators in the filename, the format guesser is bypassed, and the video handler defaults to YUV_420 (the constructor fallback). This forces users to manually switch the format dropdown to YUV 4:0:0 in the properties panel every time they open a clean .raw file.

Additionally, in checkSpecificFileExtensions inside PixelFormatYUVGuess.cpp, there is a potential crash hazard where guessedFrameFormat.frameSize is dereferenced without checking if it has a value.

To Reproduce

Steps to reproduce the behavior:

1. Open a raw video file named 1.raw (which does not contain resolution indicators like _1920x1080_ in its filename).
2. Look at the properties panel.
3. See that the pixel format defaults to YUV 4:2:0.

Expected behavior

The .raw file should default to YUV 4:0:0 (YUV_400 luma-only) format upon loading, allowing the user to only manually input the width/height in the properties GUI.

Screenshots

N/A

Version

• OS: Windows / macOS / Linux
• Version: develop branch, commit a72eb34 or later
• If you compiled YUView yourself then which Qt version did you use? Qt 6.x