glthread: align the batch buffer to 8 bytes for pointers and doubles again

This was changed when I switched to types from size_t to int.

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4251>

src/mesa/main/glthread.h

@@ -71,6 +71,11 @@ struct glthread_batch
    int used;
 
    /** Data contained in the command buffer. */
+#ifdef _MSC_VER
+   __declspec(align(8))
+#else
+   __attribute__((aligned(8)))
+#endif
    uint8_t buffer[MARSHAL_MAX_CMD_SIZE];
 };