[rlgl] Implementing rlReloadVertexBuffer() and rlReloadVertexBufferElement()

[mikeemm]

Currently, a VBO/EBO cannot be resized after being loaded with rlLoadVertexBuffer() or its element counterpart. Using rlUpdateVertexBuffer() only works if your new vertex data is as big or smaller than what was initially allocated; not if it is bigger. The buffer's usage is also not modifiable this way.

At the moment, the raylib-only solution is to use rlUnloadVertexBuffer on the old buffer and load it anew with rlLoadVertexBuffer(), while also recreating any VAOs using these VBO/EBOs. Implementing a solution within your own code doesn't seem to be possible out of the box considering that the OpenGL functions it requires don't seem to be exposed by Raylib.

With these changes we get rlReloadVertexBuffer() and rlReloadVertexBufferElement(), which reload an existing vertex buffer without requiring new bindings. I believe this is a really simple addition that can be really helpful in cases when you may want to modify parts of an existing mesh without destroying and recreating it completely, or when you're handling vertex buffer objects on your own without using Raylib's structures directly.