Add flag to RTC RC config for screen content

Currently not being used / implemented / tested.
Implementation and tests will follow.

Bug: b/281463780
Change-Id: Ia1f84e1420f51963ab1cd0ea1f8410594b31f3d4

av1/ratectrl_rtc.h

diff --git a/av1/ratectrl_rtc.h b/av1/ratectrl_rtc.h
index c5d0caa..6c223a7 100644
--- a/av1/ratectrl_rtc.h
+++ b/av1/ratectrl_rtc.h
@@ -32,6 +32,8 @@
 
   int width;
   int height;
+  // Flag indicating if the content is screen or not.
+  bool is_screen;
   // 0-63
   int max_quantizer;
   int min_quantizer;