egl/dri2: add image extension such it's usable by swrast driver

egl/dri2: add image extension such it's usable by swrast driver

Otherwise, this extension is not visible to the EGL users who
use the swrast driver.

This will allow the swrast driver to use eglCreateImageKHR,
provided the target is EGL_GL_TEXTURE_2D_KHR or
EGL_GL_RENDERBUFFER_KHR.  Note we still have to implement the
create from render buffer path.

v2: add it to optional_core_extensions instead of swrast_core_extensions,
    so it's not a requirement (Emil)
v3: Merge egl/dri2 changes together, also add support for
    platform_wayland (Emil)

Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com> (v2)