Fixes #280 - Fix -Wswitch errors in src/libwebvtt/cuetext.c

-Replaced 'switch' statement on line 178 with conditional branches

-Replaced 'switch' statement on line 693 with conditional branches

N'either of these cases seem to really require a switch statement as
there are only 2-3 possible scenarios in each of them.

-Moved conditional branch comments (lines 701, 706, 710, 718) to within the
braces, as requested by @rillian.