Add crop feature

Add an option to crop the screen on the server. This allows to mirror
only part of the device screen.

app/src/scrcpy.h

@@ -5,6 +5,7 @@
 
 struct scrcpy_options {
     const char *serial;
+    const char *crop;
     Uint16 port;
     Uint16 max_size;
     Uint32 bit_rate;