Revert "Revert WebP support due to broken builds"

This reverts commit 6709bdcacd.

There was an issue with the script that removes '-lwebp' from WebP
linker flags, since we're statically linking. This is now fixed.

platform/default/webp_reader.cpp

@@ -0,0 +1,27 @@
+#include <mbgl/util/image.hpp>
+#include <mbgl/util/premultiply.hpp>
+#include <mbgl/platform/log.hpp>
+
+extern "C"
+{
+#include <webp/decode.h>
+}
+
+namespace mbgl {
+
+PremultipliedImage decodeWebP(const uint8_t* data, size_t size) {
+    int width = 0, height = 0;
+    if (WebPGetInfo(data, size, &width, &height) == 0) {
+        throw std::runtime_error("failed to retrieve WebP basic header information");
+    }
+
+    std::unique_ptr<uint8_t[]> webp(WebPDecodeRGBA(data, size, &width, &height));
+    if (!webp) {
+        throw std::runtime_error("failed to decode WebP data");
+    }
+
+    UnassociatedImage image { size_t(width), size_t(height), std::move(webp) };
+    return util::premultiply(std::move(image));
+}
+
+} // namespace mbgl