native: don't build egl directory by default

`egl` is only used for reverse engineering, and is not necessary
for using etnaviv.
Use 'make rev' to build the reverse-engineering only directories.
driver: move RS flush to more logical place
demos: remove dependency on GL headers

GLES/EGL headers were only used for data types. Use native types instead
to make it easier to build etnaviv for targets for which the rootfs
provides no GL headers.