Don't define Texture3D on WebGL 1.0.

There's no matching extension (no OES_texture3D equivalent), moreover
constructing Texture3D doesn't even link (missing definitions).

src/Magnum/Magnum.h

@@ -639,7 +639,9 @@ template<UnsignedInt> class Texture;
 typedef Texture<1> Texture1D;
 #endif
 typedef Texture<2> Texture2D;
+#if !(defined(MAGNUM_TARGET_WEBGL) && defined(MAGNUM_TARGET_GLES2))
 typedef Texture<3> Texture3D;
+#endif
 
 #ifndef MAGNUM_TARGET_GLES2
 template<UnsignedInt> class TextureArray;

src/Magnum/Texture.h

@@ -1279,6 +1279,7 @@ typedef Texture<1> Texture1D;
 /** @brief Two-dimensional texture */
 typedef Texture<2> Texture2D;
 
+#if !(defined(MAGNUM_TARGET_WEBGL) && defined(MAGNUM_TARGET_GLES2))
 /**
 @brief Three-dimensional texture
 
@@ -1286,6 +1287,7 @@ typedef Texture<2> Texture2D;
 @requires_webgl20 3D textures are not available in WebGL 1.0.
 */
 typedef Texture<3> Texture3D;
+#endif
 
 }