Compressed Image Extensions
When using glad and compressed DXT1 textures, I get an error that `GL_COMPRESSED_RGB_S3TC_DXT1_EXT` is undefined. However, my system does support them and if I manually add the following (taken from glcorearb.h), my program works fine:

```
#ifndef GL_EXT_texture_compression_s3tc
#define GL_EXT_texture_compression_s3tc 1
#define GL_COMPRESSED_RGB_S3TC_DXT1_EXT   0x83F0
#define GL_COMPRESSED_RGBA_S3TC_DXT1_EXT  0x83F1
#define GL_COMPRESSED_RGBA_S3TC_DXT3_EXT  0x83F2
#define GL_COMPRESSED_RGBA_S3TC_DXT5_EXT  0x83F3
#endif
```

Is their a reason these are not defined in glad?