config: tweak.surface-bit-depth: add 16-bit

This adds support for 16-bit (integer) surfaces. The corresponding
pixman type (PIXMAN_a16b16g16r16) was added in 0.46.0.

If the new 16-bit type is requested, but not supported by the
compositor, fallback to 16f-bit, and if that fails, 10-bit (and
finally 8-bit).

config.h

@@ -199,6 +199,7 @@ enum shm_bit_depth {
     SHM_BITS_AUTO,
     SHM_BITS_8,
     SHM_BITS_10,
+    SHM_BITS_16,
     SHM_BITS_16F,
 };