TGA: Support for TGA specification 2.0

Adds TGA 2.0 compliance:
- Support for Extension Area, Developer Area and Footer (metadata support)
- Support for 15-bit and 16-bit per pixel images (both RGB and Indexed)
- Full support for rotation on reading (we cannot use Qt transformations because only a subset is part of the TGA specification)
- When writing you can choose the supported version (subType)
- Improved writing speed (approximately 10 times) and removed whole image conversions (significant memory savings)

It pass the [TrueVision TGA 2.0 conformance suite](https://github.com/zigimg/test-suite/tree/master/fixtures/tga).

Test changes:
- Read test: added ability to skip a specific test on sequential devices (via JSON behavior file)
- Write test: added the ability to set the subType when writing (via JSON properties file)

Closes #37

autotests/writetest.cpp

@@ -70,6 +70,15 @@ void setOptionalInfo(QImage &image, const QString &suffix)
     }
 }
 
+QByteArray readSubType(const QString &suffix)
+{
+    auto obj = readOptionalInfo(suffix);
+    if (obj.isEmpty()) {
+        return {};
+    }
+    return obj.value("subType").toString().toLatin1();
+}
+
 bool checkOptionalInfo(QImage &image, const QString &suffix)
 {
     auto obj = readOptionalInfo(suffix);
@@ -157,6 +166,9 @@ int basicTest(const QString &suffix, bool lossless, bool ignoreDataCheck, bool s
         {
             QBuffer buffer(&writtenData);
             QImageWriter imgWriter(&buffer, format.constData());
+            auto subType = readSubType(suffix);
+            if (!subType.isEmpty())
+                imgWriter.setSubType(subType);
             if (lossless) {
                 imgWriter.setQuality(100);
             }