Added IndexBuffer, added cpy ctor and = overload to texture and
indexbuffer and renamed vertexarray to vertexarrayobject to avoid
confusion about its purpose. Also added some convinience functions and
enum values for tesselation shaders and made the code base a bit more
const correct. VertexAttributeArray and UniformBuffer still need to get a
copy ctor and assignment overload. Will do that next. A cameraclass and
framebuffer are also in the planning for this week.