Mask Corner Radius
mask=corners is set on an image to create rounded corners,
corner-radius specifies the width of the radius in pixels.
A single value applies the same radius to all four corners, or comma-separated values can be specified to control each corner individually, starting with the top left. This makes common use cases such as only rounding the top or bottom simple to implement.
corner-radius isn’t set, the radius defaults to 20% of the smaller dimension (width or height) for all corners. The masked area defaults to transparent (PNG output only) or white.
maskbg is also applied, the masked area will fill in with the specified color.
See the API specification for more information about this parameter's value ranges and format.