examples: warn on invalid numeric parameters

add ExUtilGet[U]Int / ExUtilGetFloat which print an error message on
parse failure.
fixes issue #219.

Change-Id: Ie537f5aebd138925bf1a48289b6b5e261b3af2ca
fix loop bug in DispatchAlpha()

* We were re-doing most of the work in plain-C as 'left-over'.
* we were always returning has_alpha = true because of a bad mask all_0xff

These bugs were conservative and silent, in the sense that we were 'just' doing
more work than necessary.

Now, the SSE2 version is really 2x faster than the C version.

Change-Id: I6c8132a267fe3c7a3d1fa70e7a5fcd10719543fa
gif2webp: Handle frames with missing  graphic control extension

According to the GIF spec (http://www.w3.org/Graphics/GIF/spec-gif89a.txt),
this block is optional, and its scope is only the first graphic rendering block
that follows.

The spec doesn't mention what default values of frame dimensions, offsets,
duration and transparent index to use in such a case, though. So, we use the
defaults used by GIF reader in Chromium:
https://code.google.com/p/chromium/codesearch#chromium/src/third_party/WebKit/Source/platform/image-decoders/gif/GIFImageReader.h&l=186

Change-Id: Iecc6967847192483770e85ac15fe2835cd01ce7b
Merge "fix loop bug in DispatchAlpha()"
webpmux: fix loop_count range check

explicitly check [0, 65535], the use of 'long' was removed in a prior
commit

Change-Id: I70d5bf286908459b5d4d619c657853f0e833b6ea
Merge "examples: warn on invalid numeric parameters"
Merge "webpmux: fix loop_count range check"