Fix [-Wimplicit-fallthrough] warnings in GD extension

And fix indentation issue

Author: Girgias

ext/gd/libgd/gd_interpolation.c

@@ -2465,6 +2465,7 @@ int gdImageSetInterpolationMethod(gdImagePtr im, gdInterpolationMethod id)
 	switch (id) {
 		case GD_DEFAULT:
 			id = GD_BILINEAR_FIXED;
+			fallthrough;
 		/* Optimized versions */
 		case GD_BILINEAR_FIXED:
 		case GD_BICUBIC_FIXED:

ext/gd/libgd/gd_png.c

@@ -309,26 +309,26 @@ gdImagePtr gdImageCreateFromPngCtx (gdIOCtx * infile)
 
 		case PNG_COLOR_TYPE_GRAY_ALPHA:
 			png_set_gray_to_rgb(png_ptr);
-
-			case PNG_COLOR_TYPE_RGB:
-			case PNG_COLOR_TYPE_RGB_ALPHA:
-				/* gd 2.0: we now support truecolor. See the comment above
-				 * for a rare situation in which the transparent pixel may not
-				 * work properly with 16-bit channels.
-				 */
-				if (png_get_valid(png_ptr, info_ptr, PNG_INFO_tRNS)) {
-					png_get_tRNS(png_ptr, info_ptr, NULL, NULL, &trans_color_rgb);
-					if (bit_depth == 16) { /* png_set_strip_16() not yet in effect */
-						transparent = gdTrueColor(trans_color_rgb->red >> 8,
-									trans_color_rgb->green >> 8,
-									trans_color_rgb->blue >> 8);
-					} else {
-						transparent = gdTrueColor(trans_color_rgb->red,
-									trans_color_rgb->green,
-									trans_color_rgb->blue);
-					}
+			fallthrough;
+		case PNG_COLOR_TYPE_RGB:
+		case PNG_COLOR_TYPE_RGB_ALPHA:
+			/* gd 2.0: we now support truecolor. See the comment above
+			 * for a rare situation in which the transparent pixel may not
+			 * work properly with 16-bit channels.
+			 */
+			if (png_get_valid(png_ptr, info_ptr, PNG_INFO_tRNS)) {
+				png_get_tRNS(png_ptr, info_ptr, NULL, NULL, &trans_color_rgb);
+				if (bit_depth == 16) { /* png_set_strip_16() not yet in effect */
+					transparent = gdTrueColor(trans_color_rgb->red >> 8,
+								trans_color_rgb->green >> 8,
+								trans_color_rgb->blue >> 8);
+				} else {
+					transparent = gdTrueColor(trans_color_rgb->red,
+								trans_color_rgb->green,
+								trans_color_rgb->blue);
 				}
-				break;
+			}
+			break;
 	}
 
 	png_read_update_info(png_ptr, info_ptr);