Let IcoEncoder, CurEncoder and AniEncoder choose the PNG filter for PNG frames

[Der-Floh]

Follow-up to #3213, as suggested there.

Problem

IcoEncoder, CurEncoder and AniEncoder create the PngEncoder for PNG-compressed frames inside IconEncoderCore, with fixed settings. FilterMethod is never set, so every PNG frame uses the default filter, Paeth. PngEncoder exposes FilterMethod, but the icon encoders offer no way to pass a different one.

Why it matters

Which filter compresses best depends a lot on the artwork, and icon and cursor art (flat colours, anti-aliased edges, large transparent areas) often does much better with something other than Paeth. These are 2,070 real cursor images (32–256 px), re-encoded with PngEncoder with only FilterMethod changed:

Filter	Total size
Paeth (what the icon encoders write today)	7,931,881 B
Adaptive	7,269,124 B (−8.4 %)
None	5,732,551 B (−27.7 %)

Since no single filter is best for every image, the caller should be able to choose.

Proposal

Add an optional filter setting for PNG frames to the icon encoders and pass it to the PngEncoder that IconEncoderCore creates:

image.Save(stream, new CurEncoder { PngFilterMethod = PngFilterMethod.None });

• Default null keeps today's behaviour, so existing output stays byte-identical.
• The three encoders share IconEncoderCore, so one setting covers ICO, CUR and ANI.
• Only frames with IconFrameCompression.Png are affected; BMP frames don't change.

An alternative would be to accept a configured PngEncoder for PNG frames, which would also expose settings such as CompressionLevel.