st/mesa: expose GL_OES_texture_view

For format fallbacks like ETC and ASTC, switching between sRGB and linear
decoding is undefined, or at least is not bit-exact. Same as
EXT_texture_sRGB_decode on GLES.

There are no piglit or dEQP regresssions.

Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>

docs/relnotes/19.0.0.html

@@ -40,6 +40,7 @@ TBD.
 
 <ul>
 <li>GL_EXT_shader_implicit_conversions on all drivers (ES extension).</li>
+<li>GL_OES_texture_view on drivers supporting texture views (ES extension).</li>
 </ul>
 
 <h2>Bug fixes</h2>