Simplify WBMP imagecreatefromstring() detection

[cmb69]

According to the WBMP specification[1], the first field (type) of a WBMP is a multi-byte integer, but only type 0 is supported. Thus there is no need to read a multi-byte integer. The second field (fix header) is a single byte; reading a multi-byte integer is not really wrong, since the fix header field is laid out in a way which allows it to be treated as such, but the check whether the MBI is greater than or equal to zero is pretty useless, because negative values could only be returned if overflow occurs (MBIs are unsigned).

So the only useful assumption we can make is that the first byte is zero; we let gdImageCreateFromWBMPCtx() figure out the rest.

[1] https://www.wapforum.org/what/technical/SPEC-WAESpec-19990524.pdf section 6

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At first I wanted to use src.wbmp for the test but it failed due to

php-src/ext/gd/gd.c

Lines 1386 to 1389 in b948332

	if (ZSTR_LEN(data) < 12) {
	/* Handle this the same way as an unknown image type. */
	return -1;
	}

src.wbmp is only 9 bytes. We could fix this, but I don't think it's worth it, because such small WBMPs likely don't exist in practice (and even larger WBMPs likely are no longer relevant).

[devnexen]

I guess good riddance ? LGTM