fix(cliprdr): accept CF_DIB with BITFIELDS compression and standard masks

[Marynych Oleksandr (maryny4)]

Windows commonly puts 32bpp CF_DIB on the clipboard with BI_BITFIELDS compression and the standard BGRA color masks. The CF_DIBV5 path already accepts that and validates the masks, but the CF_DIB path rejected every compression other than BI_RGB, so dib_to_png failed and consumers such as ironrdp-web and ironrdp-daemon dropped the image; a downstream project has to rewrite the header to BI_RGB to paste it at all.

With the standard masks the pixels are laid out exactly as with BI_RGB, so once the masks are validated they decode the same way. This moves the mask validation shared with BITMAPV5HEADER into one helper, reads the three masks that follow a BITFIELDS BITMAPINFOHEADER (the V1 layout carries no alpha mask) and accounts for them in the logical length reported by validate_dib. Non-standard mask orders stay rejected on both paths, as before.

Tests build the BITFIELDS variants from the existing cf_dib.pdu and cf_dibv5.pdu fixtures. A V1 BITFIELDS DIB with standard masks decodes to a PNG byte-identical to the BI_RGB fixture and validate_dib reports the full length including the masks; a non-standard mask order is refused; masks cut off mid-header are a decode error rather than a panic; and the V5 path keeps its previous behavior through the shared helper.