egl/gl: Add an example of dma-buf texture sharing

The dmabufshare executable forks to create two processes, server and client. The server creates a texture and shares it with the client. Sharing is done with specific extensions from EGL and GL that use DMA-BUF. The server periodically updates the texture to show that both the server and the client reference the same texture.

This is a port of my dma-buf texture sharing example from https://gitlab.com/blaztinn/dma-buf-texture-sharing The difference from the original example is that it is contained inside a single file, uses eglut and automatically forks (to avoid running manually two processes).