Fix some GPU image filter code to preserve precision and color space

src/gpu/GrDrawingManager.cpp

Index: src/gpu/GrDrawingManager.cpp
diff --git a/src/gpu/GrDrawingManager.cpp b/src/gpu/GrDrawingManager.cpp
index 606a9abd6e43c3530661e3e2c28e3949cdac6ed6..9dd5613f21bd54a3aa7786b9ee8961054521d73c 100644
--- a/src/gpu/GrDrawingManager.cpp
+++ b/src/gpu/GrDrawingManager.cpp
@@ -222,6 +222,8 @@ sk_sp<GrDrawContext> GrDrawingManager::makeDrawContext(sk_sp<GrRenderTarget> rt,
     // by, including internal usage. We allow a null color space here, for read/write pixels and
     // other special code paths. If a color space is provided, though, enforce all other rules.
     if (colorSpace && !SkSurface_Gpu::Valid(fContext, rt->config(), colorSpace.get())) {
+        // SRGBTODO: Enable this assert once image filters are propagating color type and space
+//        SkDEBUGFAIL("Invalid config and colorspace combination");
         return nullptr;
     }