feat(filter): get output dimensions from filter graph directly

[wizyoung]

Previously, output dimensions were derived by manually parsing the filter string. This approach was fragile and failed to handle more complex FFmpeg filter expressions, such as:

• scale='min(720,iw)':'min(720,ih)'
• scale=iw/2:ih/2
• force_original_aspect_ratio=decrease
• pad / crop filters with expressions

This change removes the string-based inference and instead queries the validated filter graph directly via av_buffersink_get_w/h to obtain the actual output dimensions. This approach is more accurate, correctly handles cases with rotation metadata, and fully supports the complete range of FFmpeg filter syntax.

One caveat is that yuv420p requires both dimensions to be even. To make this explicit and avoid silent misbehavior, an error is now returned to the user when this constraint is violated.

I have tested the following filter configurations, all of which now work correctly with the new approach:

# Scale to half size. round down to even
filter="format=yuv420p,scale=w='trunc(iw/4)*2':h='trunc(ih/4)*2'"
# round up to even
filter="format=yuv420p,scale=w='ceil(iw/4)*2':h='ceil(ih/4)*2'"

# Scale width to 256, height auto (preserve aspect ratio)
filter="format=yuv420p,scale=w=256:h=-2:flags=bilinear"

# Set shorter side to 480, preserve aspect ratio, force even
filter="format=yuv420p,scale='if(lt(iw,ih),480,-2)':'if(lt(iw,ih),-2,480)'"

# Pad to square (letterbox), using custom padding color. 
filter="format=yuv420p,scale='trunc(iw/2)*2':'trunc(ih/2)*2',pad=w=max(iw\\,ih):h=max(iw\\,ih):x=(ow-iw)/2:y=(oh-ih)/2:color=0x688082"

# Limit longer side to 720, then pad to square
filter="format=yuv420p,scale='min(720,iw)':'min(720,ih)':force_original_aspect_ratio=decrease:force_divisible_by=2,pad=w=max(iw\\,ih):h=max(iw\\,ih):x=(ow-iw)/2:y=(oh-ih)/2"

# Center crop to 480x480 square
filter="format=yuv420p,crop=w=480:h=480:x=(iw-480)/2:y=(ih-480)/2"

[gcanat]

Nice! LGTM. ✌️