libgui: Enable -Weverything and -Werror

Enables -Weverything and -Werror, with just a few exceptions for
warnings we can't (or shouldn't need to) work around.

This is a squashed commit based on an initial change with a couple of
fixes to avoid breaking certain targets. The source commits are:
  d723bd7669b4fc88dc282d8bf8ba5ecb2849d22f
  00d504c06ea034befe143e6b8cb34d004670ed02
  429ba89cd293633d2f882165066b422c96597df2

Change-Id: I034abec27bf4020d84af60d7acc1939c59986dd6
diff --git a/libs/gui/StreamSplitter.cpp b/libs/gui/StreamSplitter.cpp
index 2fd09eb..4a1f9e0 100644
--- a/libs/gui/StreamSplitter.cpp
+++ b/libs/gui/StreamSplitter.cpp
@@ -141,7 +141,7 @@
 
     IGraphicBufferProducer::QueueBufferInput queueInput(
             bufferItem.mTimestamp, bufferItem.mIsAutoTimestamp,
-            bufferItem.mCrop, bufferItem.mScalingMode,
+            bufferItem.mCrop, static_cast<int32_t>(bufferItem.mScalingMode),
             bufferItem.mTransform, bufferItem.mIsDroppable,
             bufferItem.mFence);