Improved PHP5 compatibility for the image extension.

Summary: * Added ICO support to `getimagesize()`. This fixes #2985.
 * Added BITMAPV5HEADER support to `getimagesize()`.
 * Removed superfluous warning in `getimagesize()`.
 * Fixed handling of `imageinfo` argument in `getimagesize()`.
 * Fixed the bmp mimetype.
 * Added missing test fixtures.
Closes #3080

Reviewed By: @LiraNuna

Differential Revision: D1427298