Use glm as subtree and move glad to be a subtree

[9prady9]

Change(s)

• Replace submodules with subtree of glm, glad repositories
• Add glad interface target to forge export targets when forge is built as static library.

Fixes #212

Current Behavior

• glm is included as submodule
• glad is included as regular directory/files

New Behavior

• glm is included as subtree under extern/glm
• glad is included as subtree under extern/glad

Breaking Change(s)

No breaking API changes.

Please check if the PR fulfills these requirements

• [x] Guidelines
• ~[ ] Relevant Examples are added/modified~
• ~[ ] Relevant Documentation has been added/updated~

Conditional Checks

• ~[ ] Please make sure SDL toolkit changes are tested as CI right now uses GLFW only.~

Other Information