Also implement AsUniformValue for Texture2d and friends directly

In addition to implementing if for references, i.e.
`impl<'a> AsUniformValue for &'a {myname}`.

The method `as_uniform_value(&self)` takes self by reference, so there
should be no need to require the second-level reference.

This allows a longer lifetime to be automatically inferred. Fixes
compilation of the texture_2d_as_uniform_value_lifetime() test from the
previous commit.

This commit doesn't remove the original impl above in order not to introduce
breaking changes. Removing would mean every texture passing in `uniform! {}`
would have to change.

build/textures.rs

@@ -425,6 +425,13 @@ fn build_texture<W: Write>(dest: &mut W, ty: TextureType, dimensions: TextureDim
                                 }}
                             }}
 
+                            impl AsUniformValue for {myname} {{
+                                #[inline]
+                                fn as_uniform_value(&self) -> UniformValue {{
+                                    UniformValue::{myname}(self, None)
+                                }}
+                            }}
+
                             impl<'a> AsUniformValue for Sampler<'a, {myname}> {{
                                 #[inline]
                                 fn as_uniform_value(&self) -> UniformValue {{