Stop trying to assess the image dimensions of large files and file chunks

Summary:
Depends on D18828. Ref T7789. See https://discourse.phabricator-community.org/t/git-lfs-fails-with-large-images/584.

Currently, when you upload a large (>4MB) image, we may try to assess the dimensions for the image and for each individual chunk.

At best, this is slow and not useful. At worst, it fatals or consumes a ton of memory and I/O we don't need to be using.

Instead:

• Don't try to assess dimensions for chunked files.
• Don't try to assess dimensions for the chunks themselves.
• Squelch errors for bad data, etc., that gd can't actually read, since we recover sensibly.

Test Plan:

• Created a 2048x2048 PNG in Photoshop using the "Random Noise" filter which weighs 8.5MB.
• Uploaded it.
• Before patch: got complaints in log about imagecreatefromstring() failing, although the actual upload went OK in my environment.
• After patch: clean log, no attempt to detect the size of a big image.
• Also uploaded a small image, got dimensions detected properly still.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T7789

Differential Revision: https://secure.phabricator.com/D18830