Add flag to disable double buffering

Some devices don't support double buffering, so add
the flag TW_DISABLE_DOUBLE_BUFFERING to disable it

Change-Id: Ia9c233fa229ae9c221a6c6f219b216d1753052f4
diff --git a/minuitwrp/graphics.c b/minuitwrp/graphics.c
index 704389f..04c41fb 100644
--- a/minuitwrp/graphics.c
+++ b/minuitwrp/graphics.c
@@ -295,8 +295,12 @@
 
     fb++;
 
+#ifndef TW_DISABLE_DOUBLE_BUFFERING
     /* check if we can use double buffering */
     if (vi.yres * fi.line_length * 2 > fi.smem_len)
+#else
+    printf("TW_DISABLE_DOUBLE_BUFFERING := true\n");
+#endif
         return fd;
 
     double_buffering = 1;