TGA: memory optimizations and native grayscale support

- Removed temporary buffer of uncompressed image size when reading (loads images using half the memory)
- Added native support for Grayscale images (they are no longer converted to RGB(A) 32 when reading/writing)
- Fixes wrong X channel value on RGBX32 images (CCBUG: 499584)

It should also reduce loading times for corrupted images.

Closes #33

src/imageformats/tga_p.h

@@ -29,6 +29,8 @@ public:
 private:
     bool writeIndexed(const QImage &image);
 
+    bool writeGrayscale(const QImage &image);
+
     bool writeRGBA(const QImage &image);
 
     const QScopedPointer<TGAHandlerPrivate> d;