Added OpenGL | ES 2.0 extensions functions

|SOURCE|
[Common/GLES2Extensions]
* Added glDeleteVertexBuffers and InitialiseGLES2Extensions to get the address
  for the GLES function
[Common/PolygonCache]
* Purge should delete all resources from the GPU
[Common/Game]
* InitialiseGLES2Extensions added after the renderer is initialised