Feat: implement MG_Impl/Buffer
Feat: implement MG_State/ErrorState and integrate it into MG_State/GLContext
Feat: add buffer deletion management
Feat: introduce `INSERTION_POINT` (https://github.com/MobileGL-Dev/MobileGLCodeManager)
Feat: add tests about MG_Impl/Buffer and buffer deletion feature
Feat: add several converters
Fix: fix some compiling errors
Fix: fix some incorrect behaviors in MG_State/Buffer
Improvement: remove some current statements at the beginning of the source file, which will be added uniformly soon
Improvement: optimize some naming
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