PHLEGM v9 r28 — PNG screenshot testing

1. Compile PHLEGM on Windows 32/64 OpenGL/D3D11 and Linux x86/x86_64.
2. Load a graphical machine; run SCREENSHOT 0; check screenshots/*.png.
3. Verify first eight bytes are: 89 50 4E 47 0D 0A 1A 0A.
4. Open with PNG viewer. Check colors, orientation, resolution and frame.
5. Use SCREENSHOT 0 test.png and SCREENSHOT 0 legacy.bmp; the second
   command must save legacy.png instead of writing BMP or legacy.bmp.png.
6. Test GUI automatic and explicit screenshots and ActRaiser timer captures.
7. Take repeated captures on a frozen frame; names must not collide.
8. Confirm custom target overwrite behavior and no leftover .png.tmp files.
9. Confirm existing BMP files are preserved and r27 ActRaiser ROM path works.
10. Validate CRCs / decompression and images with independent PNG decoder.

PNG format: IHDR 8-bit RGB, filter 0, IDAT with zlib-wrapped stored DEFLATE,
IEND, PNG CRC32, Adler32. Compression ratio is not the focus of this revision.
