Add an option in mjrContext for mjr_readPixels output format.

PiperOrigin-RevId: 500342924
Change-Id: I43e8c6208db341e2bdbef10382b7293c35bdca65

src/render/render_gl2.c

@@ -153,7 +153,7 @@ void mjr_readPixels(unsigned char* rgb, float* depth,
     // read rgb and depth
     if (rgb) {
       glReadPixels(viewport.left, viewport.bottom, viewport.width, viewport.height,
-                   GL_RGB, GL_UNSIGNED_BYTE, rgb);
+                   con->readPixelFormat, GL_UNSIGNED_BYTE, rgb);
     }
     if (depth) {
       glReadPixels(viewport.left, viewport.bottom, viewport.width, viewport.height,