Do target_type changes only in toktx (KhronosGroup#757)

They were being done in both `toktx` and the `imageinput` plugin.

Fixes KhronosGroup#727.

* Move image-related utilities and image.hpp from `ktxtools` to `imageio` target for sharing
   between new and legacy tools.
* Change `toktx` to use the updated version of image.hpp.
* Make `other_Includes` a system include for `imageio` and `unittests`.
* Make `toktx`  use `imageio`'s INTERFACE includes instead of directly referencing the directories.
* Fix mismatched `new[]` and `delete` in `glloadtests`.

CMakeLists.txt

@@ -505,13 +505,18 @@ macro(common_libktx_settings target enable_write library_type)
         $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/lib/basisu/zstd>
         $<INSTALL_INTERFACE:lib/basisu/zstd>
 
-        $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/other_include>
-        $<INSTALL_INTERFACE:other_include>
-
         $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/utils>
         $<INSTALL_INTERFACE:utils>
     )
 
+    target_include_directories(
+        ${target}
+        SYSTEM
+    PRIVATE
+        $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/other_include>
+        $<INSTALL_INTERFACE:other_include>
+    )
+
     if( ${library_type} STREQUAL STATIC )
         target_compile_definitions(${target} PUBLIC KHRONOS_STATIC)
     endif()