Add an option in mjrContext for mjr_readPixels output format.

PiperOrigin-RevId: 500342924
Change-Id: I43e8c6208db341e2bdbef10382b7293c35bdca65

src/render/render_context.c

@@ -1602,6 +1602,9 @@ void mjr_makeContext_offSize(const mjModel* m, mjrContext* con, int fontscale,
   while ((err = glGetError())) {
     mju_warning_i("OpenGL error 0x%x in or before mjr_makeContext", err);
   }
+
+  // set default color pixel format for mjr_readPixels
+  con->readPixelFormat = GL_RGB;
 }