Disable modifying offsets of all frames of sprite

Instead of disabling modifying of offsets if ANY sprites are merged from
a PWAD (commit 5ed8dd00cb3e9afb4d3081d2c53df379f66b683b), when merging a
sprite, disable modifying of offsets of all other frames. This means
Revenant100's Doom 2 Minor Sprite Fixing Project will behave.