SCI: Improve MidiParser_SCI robustness against bad sound resources

1. KQ4 sound 104 has an extra 0xFC (MIDI Stop command/kEndOfTrack)
   at the end of the resource, which causes an out-of-bounds read
   because the filtering loop continues after the first 0xFC and
   unconditionally attempts to read 2 bytes (expecting there to
   always be a delta value + a command, whereas in this file there
   is only another kEndOfTrack command). This is corrected by
   exiting the filtering loop when a kEndOfTrack is encountered
   and there is not enough data remaining in the resource to
   continue reading.

2. KQ5 sound 699 is truncated, which causes the parser to attempt
   to read past the end of the resource. This is addressed by
   adding bounds checks that exit the mix loop early if there is
   no more data available to read. This allows truncated sounds
   to be played as far as possible (previously, trying to read
   truncated resources would result in a fatal error).

3. midiMixChannels allocates an arbitrary amount of raw memory
   for the mixed MIDI sequence, without performing any bounds
   checking when writing to this memory, potentially leading to
   a crash or silent corruption of adjacent memory. This is
   mitigated by using SciSpan instead of a raw pointer for the
   mixed data.

Fixes Trac#9727.

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