sokol: add explicit casts for g++ compatability (#26744)

* sokol: add explicit casts for g++ compatability

* fix: use Backend struct instead of int

Author: dy-tea

vlib/sokol/gfx/gfx.c.v

@@ -179,7 +179,9 @@ pub fn query_desc() Desc {
 
 @[inline]
 pub fn query_backend() Backend {
-	return C.sg_query_backend()
+	unsafe {
+		return Backend(C.sg_query_backend())
+	}
 }
 
 @[inline]

vlib/sokol/sapp/sapp_structs.c.v

@@ -301,7 +301,7 @@ pub mut:
 pub type Event = C.sapp_event
 
 pub fn (e &Event) str() string {
-	return 'evt: frame_count=${e.frame_count}, type=${e.type}'
+	return 'evt: frame_count=${e.frame_count}, type=${EventType(e.type)}'
 }
 
 @[typedef]