Use cached tiles while doing a if-none-modified request

[kkaefer]

We should show cached, but expired tiles immediately, do a If-None-Modified request meanwhile and update tile if changed. While this technically violates the HTTP spec, it will also ensure that the user sees a map as soon as possible. In most cases, the map is either fresh, or the visible differences are negligible.

[aruediger]

It's either If-None-Match (ETag based) or If-Modified-Since (time based). Just sayin.

[kkaefer]

@2Beaucoup you're right, and the code uses the correct header names:

mapbox-gl-native/platform/default/http_request_curl.cpp

Lines 341 to 348 in a3ea4ac

	if (!existingResponse->etag.empty()) {
	const std::string header = std::string("If-None-Match: ") + existingResponse->etag;
	headers = curl_slist_append(headers, header.c_str());
	} else if (existingResponse->modified) {
	const std::string time =
	std::string("If-Modified-Since: ") + util::rfc1123(existingResponse->modified);
	headers = curl_slist_append(headers, time.c_str());
	}

[kkaefer]

This is implemented as part of #2617.