glthread: use int instead of size_t where it's OK

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3948>

src/mesa/main/glthread.h

@@ -60,7 +60,7 @@ struct glthread_batch
    struct gl_context *ctx;
 
    /** Amount of data used by batch commands, in bytes. */
-   size_t used;
+   int used;
 
    /** Data contained in the command buffer. */
    uint8_t buffer[MARSHAL_MAX_CMD_SIZE];