Fix crash when rendering text with hundreds of newlines

[ejud]

This is a fix to issue #312, describing a crash when hundreds of newlines exist in markdown. This appears to be device-dependent; my test device needed about 500 lines to crash, and my simulator needed about 3,000.

Diagnosis

• \n characters split the text into many inline nodes. Text of 500 lines results in 999 nodes (500 lines interlaced with 499 "soft breaks")
• TextInlineRenderer was using Text concatenation to concatenate attributed strings. Each Text + Text concatenation resulted in a nested Text object.
  • Text + Text + Text is effectively Text(Text + Text) + Text in its implementation.
• The result of this concatenation is that 500 lines creates a nested depth of somewhere around 999. At this point, a crash is likely.

Solution

TextInlineRenderer deals only with attributed strings and images. Instead of concatenating a Text view for every node, we track the sequences of attributed strings and images separately; when we append an attributed string to another attributed string, we concatenate the strings directly. Only when accessing the final result are these values converted to Text.

In effect, the nesting depth for Text objects is now proportional to the number of images inline, rather than the total number of nodes.