Prior to this change, the spacing between line N and line N + 1 would be
lineHeight (N).
This resulted in incorrect spacing when using multiple fonts in a text.
This commit uses the correct spacing, which is
maxDescent (N) + maxAscent (N + 1). This is also the same rule that was
used by TextLayout prior to JUCE 8, and the rule that CoreText's
AttributedString features are using as a general rule.
Note: lineHeight (N) = maxAscent (N) + maxDescent (N).