[Solved] Is there some reason for this kind of for-loop?

As long as you’re bothered with the for loop syntax,

 for ( ; lastheight < height ; lastheight++)

is perfectly valid, as long as lastheight is defined and initialized previously.

Quoting C11, chapter §6.8.5.3

for ( clause-1 ; expression-2 ; expression-3 ) statement

[…] Both clause-1 and expression-3 can be omitted. An omitted expression-2 is replaced by a
nonzero constant.

Regarding the reason for defining lastheight outside the for loop, one thing can be mentioned, that, for a construct like

 for ( int lastheight = 0 ; lastheight < height ; lastheight++)  {...} //C99 and above

limits the scope of lastheight to the for loop body. If you want lastheight to be used after (outside the scope of) the loop body, you must have the definition outside the loop.

Also, if my memory serves correctly, before C99, declaration of a variable was not possible inside the for statement, anyway. So, the way to go was

 int lastheight;
 for ( lastheight = 0 ; lastheight < height ; lastheight++)  {...}

Also, here’s a link to a detailed discussion about for loop syntax.

^{Disclaimer: My answer.}