Update VERSION to 10.2.6

In preparation for the 10.2.6 release.
Add release notes for the 10.2.6 release

Listing bugs fixed and changes made.
docs: Add sha256 sums for the 10.2.6 release
mesa: Handle uninitialized textures like other textures in get_tex_level_parameter_image

Instead of catching the special case early, handle it by constructing a
fake gl_texture_image that will cause the values required by the OpenGL
4.0 spec to be returned.

Previously, calling

    glGenTextures(1, &t);
    glBindTexture(GL_TEXTURE_2D, t);
    glGetTexLevelParameteriv(GL_TEXTURE_2D, 0, 0xDEADBEEF, &value);

would not generate an error.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Suggested-by: Brian Paul <brianp@vmware.com>
Cc: "10.2" <mesa-stable@lists.freedesktop.org>
Cc: Anuj Phogat <anuj.phogat@gmail.com>
(cherry picked from commit ee58c71a65bb5b769a03e4b25bd13c57a6b742d8)