Fix extraction of colour components in CPDF_DIBSource::DownSampleScanline32Bit

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 70fb380a432d88d2278a8ff67ab215d183228129..bc68804d11a45fbe8039d1df97810e2dc85b044d 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -748,6 +748,7 @@ test("pdfium_embeddertests") {
     "core/src/fpdfapi/fpdf_page/fpdf_page_func_embeddertest.cpp",
     "core/src/fpdfapi/fpdf_parser/fpdf_parser_decode_embeddertest.cpp",
     "core/src/fpdfapi/fpdf_parser/fpdf_parser_parser_embeddertest.cpp",
+    "core/src/fpdfapi/fpdf_render/fpdf_render_load_image_embeddertest.cpp",
     "core/src/fpdfapi/fpdf_render/fpdf_render_pattern_embeddertest.cpp",
     "fpdfsdk/src/fpdf_dataavail_embeddertest.cpp",
     "fpdfsdk/src/fpdfdoc_embeddertest.cpp",