Two bugs:
1. Correctly set bits in VP8X header.
Turns out these were set in the wrong order.
2. Correctly set the `has_alpha` flag.
Also add a test for writing webp files with icc data. With the
additional checks in other commits in this PR, this test catches
the bug in WebPWriter.
Rearrange some existing functions to make it easier to write this test:
* Extract encode_bitmap() from get_roundtrip_bitmap().
encode_bitmap() allows passing extra_args that the test uses to pass
in ICC data.
* Extract expect_bitmaps_equal() from test_roundtrip()
Explicit template arguments must be wrapped in parens,
else they confuse the preprocessor.
Add the parens instead of avoiding the use of explicit template
arguments.
No behavior change.
