remove bundled git sources

[jopadan]

• #99
• fix missing nonstring attribute
• make PngWrite functions static
• Use -DOSL_IMAGE_LOADER_[FORMAT] flags
• Use -DOSL_IMAGE_WRITER_[FORMAT] flags
• add format library dirs and include header path to IMG

[dogo]

Optional

Future improvement: switching from global link_libraries() / include_directories() to target_link_libraries(osl PRIVATE ...) would avoid leaking flags to consumers. Only a nice-to-have, not a blocker.

[dogo]

We no longer need the OSL_IMAGE_* macros. It's not the developer's responsibility to declare these in their projects, they are used only for internal validation of image support.