InputText: Added a line index. Refactored cursor and selection rendering, now simpler, easier to reason about, and faster. (#3237, #952, #1062, #7363)

docs/CHANGELOG.txt

@@ -66,6 +66,8 @@ Other Changes:
   In theory the buffer size should always account for a zero-terminator, but idioms
   such as using InputTextMultiline() with ImGuiInputTextFlags_ReadOnly to display
   a text blob are facilitated by allowing this.
+- InputText: refactored internals to simplify and optimizing rendering of selection.
+  Very large selection (e.g. 1 MB) now take less overhead.
 - InputText: revert a change in 1.79 where pressing Down or PageDown on the last line
   of a multi-line buffer without a trailing carriage return would keep the cursor
   unmoved. We revert back to move to the end of line in this situation.