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/templateimage.cpp

@@ -76,6 +76,15 @@ bool TemplateImage::isLicense() const
     return !m_fi.suffix().compare(QStringLiteral("license"), Qt::CaseInsensitive);
 }
 
+bool TemplateImage::skipSequentialDeviceTest() const
+{
+    auto obj = searchObject(m_fi);
+    if (obj.isEmpty()) {
+        return false;
+    }
+    return obj.value("skipSequential").toBool();
+}
+
 QFileInfo TemplateImage::compareImage(TestFlags &flags, QString& comment) const
 {
     auto fi = jsonImage(flags, comment);