fix: Crash on exit when using custom font from file

[csnover]

Problem description

WerWolv/ImHex#1814

Implementation description

1. Remove the flag that wrongly tells ImFontAtlas that it owns font data, and remove that parameter from the function signature entirely since now it is always false.
2. Rename takeAtlas to getAtlas since it no longer transfers ownership as of b652565b57bd8a9aac2779b09f51f7bac26afca2.