Add resize and scale support to create KhronosGroup#999

Missing functionality from the legacy tool added so we can comfortably remove the legacy tools in 4.5.

The following issues were discovered and fixed while adding the features:

* The target imageSpec was being updated for every input file. This
  was only working because, until now, nothing had looked at the spec
  values that were being changed once the texture had been created.
  The fix is to make separate determineSourceColorSpace and
  determineTargetColorSpace functions.
* An associated fix to the above is to move the checks for non-null
  transferFunctions to the use site with fatal errors thrown then if
  they are null. This avoids the need for second guessing to avoid
  the fatal errors when the transferFunctions will not be used. With
  this and the above fix, the code is now much more robust and clear.
* Full help was missing --levels.
* Full help had an incorrect statement regarding implicit color
  conversions dating from the transfer function work.

Author: MarkCallow

tests/cts

@@ -211,6 +211,32 @@ constexpr bool operator!=(const ImageSpec::Origin& lhs, const ImageSpec::Origin&
     return lhs.x != rhs.x || lhs.y != rhs.y || lhs.z != rhs.z;
 }
 
+[[nodiscard]] inline std::string toString(const ImageSpec::Origin& o) noexcept {
+    std::string str;
+    switch (o.x) {
+    case ImageSpec::Origin::eLeft: str = "left"; break;
+    case ImageSpec::Origin::eRight: str = "right"; break;
+    case ImageSpec::Origin::eUnspecified: str = "unspecified"; break;
+    default: assert(false && "Invalid origin.x");
+    }
+    str += ",";
+    switch (o.y) {
+    case ImageSpec::Origin::eTop: str += "top"; break;
+    case ImageSpec::Origin::eBottom: str += "bottom"; break;
+    case ImageSpec::Origin::eUnspecified: str += "unspecified"; break;
+    default: assert(false && "Invalid origin.y");
+    }
+    str += ",";
+    switch (o.z) {
+    case ImageSpec::Origin::eFront: str += "front"; break;
+    case ImageSpec::Origin::eBack: str += "back"; break;
+    case ImageSpec::Origin::eUnspecified: str += "unspecified"; break;
+    default: assert(false && "Invalid origin.z");
+    }
+
+    return str;
+}
+
 typedef std::function<void(const std::string&)> WarningCallbackFunction;
 
 enum class ImageInputFormatType {