Support for cluster splitting in multiple spans

[mta452]

This issue is a continuation of discussion in #1

Consider three characters form a ligature, but its individual characters have different color spans. The typesetter does not break runs based on color spans, but whole ligature is rendered in a single color, which is a deviation from intended behavior. The same thing happens if multiple characters form a cluster, such as marks followed by a base.

As characters are not recognizable in a ligature/cluster, they can be estimated by dividing the total advance of the cluster with the number of characters in it. Then, the whole cluster should be rendered in defined edges with clipping mode.

The end result should be similar to the following image, where the estimated portion of س and ت is rendered in red color.

[mta452]

Resolved in v2.2

[mta452]

@AbdullahRusi you may check the feature in v2.2.