Add a basic test for gdImageCreateFromWebp()

We're testing a lossless and a lossy Webp with alpha channel. The images
have been taken from <https://developers.google.com/speed/webp/gallery2>,
have been authored by Fizyplankton and are in the public domain.

The lossy test might fail for another libwebp version, so it appears to
be best to not cherry-pick into GD-2.2 yet.

tests/webp/.gitignore

@@ -1 +1,2 @@
+/basic
 /bug00111

tests/webp/CMakeLists.txt

@@ -1,5 +1,6 @@
 IF(WEBP_FOUND)
 LIST(APPEND TESTS_FILES
+	basic
 	bug00111
 )
 ENDIF(WEBP_FOUND)

tests/webp/Makemodule.am

@@ -1,7 +1,12 @@
 if HAVE_LIBWEBP
 libgd_test_programs += \
+	webp/basic \
 	webp/bug00111
 endif
 
 EXTRA_DIST += \
-	webp/CMakeLists.txt
+	webp/CMakeLists.txt \
+	webp/basic_lossless.webp \
+	webp/basic_lossless_exp.png \
+	webp/basic_lossy.webp \
+	webp/basic_lossy_exp.png

tests/webp/basic.c

@@ -0,0 +1,41 @@
+/**
+ * Basic test of gdImageCreateFromWebp()
+ *
+ * We're testing that a lossless and a lossy WebP with alpha channel are
+ * decoded as expected.
+ */
+
+
+#include "gd.h"
+#include "gdtest.h"
+
+
+static void test_image(const char *name)
+{
+    gdImagePtr im;
+    FILE *fp;
+    char buf[256];
+    char *path;
+
+    snprintf(buf, sizeof(buf), "%s.webp", name);
+    fp = gdTestFileOpen2("webp", buf);
+    im = gdImageCreateFromWebp(fp);
+    gdTestAssert(im != NULL);
+    fclose(fp);
+
+    snprintf(buf, sizeof(buf), "%s_exp.png", name);
+    path = gdTestFilePath2("webp", buf);
+    gdAssertImageEqualsToFile(path, im);
+    gdFree(path);
+
+    gdImageDestroy(im);
+}
+
+
+int main()
+{
+    test_image("basic_lossless");
+    test_image("basic_lossy");
+
+    return gdNumFailures();
+}