Disable TextureFormat::RGB10 for ES3.

It was defined in OES_required_internalformat which I don't want to
support in ES3+.

src/Magnum/AbstractTexture.cpp

@@ -455,7 +455,9 @@ ColorFormat AbstractTexture::imageFormatForInternalFormat(const TextureFormat in
         case TextureFormat::RGB5:
         #endif
         case TextureFormat::RGB565:
+        #ifndef MAGNUM_TARGET_GLES3
         case TextureFormat::RGB10:
+        #endif
         #ifndef MAGNUM_TARGET_GLES
         case TextureFormat::RGB12:
         #endif
@@ -698,7 +700,9 @@ ColorType AbstractTexture::imageTypeForInternalFormat(const TextureFormat intern
         case TextureFormat::RGB565:
             return ColorType::UnsignedShort565;
 
+        #ifndef MAGNUM_TARGET_GLES3
         case TextureFormat::RGB10:
+        #endif
         case TextureFormat::RGB10A2:
         #ifndef MAGNUM_TARGET_GLES2
         case TextureFormat::RGB10A2UI:

src/Magnum/TextureFormat.h

@@ -540,16 +540,20 @@ enum class TextureFormat: GLenum {
      */
     RGB565 = GL_RGB565,
 
+    #ifndef MAGNUM_TARGET_GLES3
     /**
      * RGB, each component normalized unsigned 10bit.
      * @requires_es_extension %Extension @es_extension{OES,required_internalformat}
-     *      and @es_extension{EXT,texture_type_2_10_10_10_REV}
+     *      and @es_extension{EXT,texture_type_2_10_10_10_REV}. Included for
+     *      compatibility reasons only, use @ref Magnum::TextureFormat::RGB10A2 "TextureFormat::RGB10A2"
+     *      in OpenGL ES 3.0 instead.
      */
     #ifndef MAGNUM_TARGET_GLES
     RGB10 = GL_RGB10,
     #else
     RGB10 = GL_RGB10_EXT,
     #endif
+    #endif
 
     #ifndef MAGNUM_TARGET_GLES
     /**