Fix sign of shift operators

This one:

  map->mask = (1 << (next_bit + bits_needed)) - (1 << next_bit);

before the fix, the shift was done as an int, causing overflow
if it ever got to 1 << 31.  Sprinkle 'u's around.

Fixes https://bugs.chromium.org/p/chromium/issues/detail?id=634805

src/hb-directwrite.cc

diff --git a/src/hb-directwrite.cc b/src/hb-directwrite.cc
index 09889d0..6846a86 100644
--- a/src/hb-directwrite.cc
+++ b/src/hb-directwrite.cc
@@ -586,7 +586,7 @@
       textString[chars_len++] = 0xFFFDu;
     else {
       textString[chars_len++] = 0xD800u + ((c - 0x10000u) >> 10);
-      textString[chars_len++] = 0xDC00u + ((c - 0x10000u) & ((1 << 10) - 1));
+      textString[chars_len++] = 0xDC00u + ((c - 0x10000u) & ((1u << 10) - 1));
     }
   }