scroll engine calculate stringsize only when scroll line updated

we calculate the size of the scrolling line every scroll update
the line data doesn't ever change without a call to update
instead calculate the string size when the line is updated
re-use this value while scrolling the line

in theory the current font could change but in practice
I can't find a case when the string size needs to be updated
that the scroll engine isn't already updating the line

if this is later an issue we can recalculate the stringsize each time
the line is finished scrolling which still saves quite a few calculations

Other:
I don't think we have any targets with pixel counts exceeding 65535 pixels
where unsigned short stringsize will be an issue

Change-Id: I83d6374377ed648d9a320d4fd69f9d6a17095b0c

firmware/export/scroll_engine.h

@@ -124,6 +124,8 @@ struct scrollinfo
     int width, height;
     /* pixel to skip from the beginning of the string, increments as the text scrolls */
     int offset;
+    /* getstringsize width of linebuffer */
+    unsigned short line_stringsize;
     /* scroll presently forward or backward? */
     bool backward;
     bool bidir;