Save both Image width and height if either is requested

Save both Image width and height if either is requested

[problem]
Too many file operations for reading image dimensions synchronously
are showing up in profiles.

[cause]
Image::GetWidth() and Image::GetHeight() each separately initiates
a file read of the image header if the image is not loaded yet.

[solution]
Modify Image::GetWidth() and Image::GetHeight() so that whichever is
called first will save both.

Signed-off-by: Andrew Cox <andrew.cox@partner.samsung.com>
Change-Id: I8116bbcb682e27eb9aaa0fe7904086df46e7699e