reinstate black-screenshot debugging code

turned off by default.

Bug: 10809349
Change-Id: I3e6b8c7860e6b0e122b8f07de4020967cd1f005c
diff --git a/services/surfaceflinger/RenderEngine/RenderEngine.cpp b/services/surfaceflinger/RenderEngine/RenderEngine.cpp
index c36fcde..ba82cad 100644
--- a/services/surfaceflinger/RenderEngine/RenderEngine.cpp
+++ b/services/surfaceflinger/RenderEngine/RenderEngine.cpp
@@ -201,6 +201,10 @@
     glDeleteTextures(count, names);
 }
 
+void RenderEngine::readPixels(size_t l, size_t b, size_t w, size_t h, uint32_t* pixels) {
+    glReadPixels(l, b, w, h, GL_RGBA, GL_UNSIGNED_BYTE, pixels);
+}
+
 void RenderEngine::dump(String8& result) {
     const GLExtensions& extensions(GLExtensions::getInstance());
     result.appendFormat("GLES: %s, %s, %s\n",