Buffer-Manager + Staging-Buffer

Basically the title... we need a Buffer-Manager with use of the Staging-Buffer for the existing Buffer-class becoming a handle only.

added 🍪 ToDo + 1 deleted label

assigned to @tfrisch

created merge request !30 (merged) to address this issue

mentioned in merge request !30 (merged)

mentioned in commit fc1a1eb6

mentioned in commit 39e6fc90

mentioned in commit cf35ba83

closed with commit f4ec5e9e

mentioned in commit f4ec5e9e

changed milestone to %Texture Cube

removed 🍪 ToDo label

added 🔥 Resource Management label