Fixed|Heretic: Time bombs spawned at incorrect height; improved look of explosions

The handling of floor clipping has changed quite a bit since vanilla
however the time bomb spawn offset had not been updated correctly.

To improve the look of the explosion I removed the 'MF_BRIGHTSHADOW'
flag applied by A_Explode and added the 'noautolight' flag to states
FIREBOMB10 and FIREBOMB11.