Reland image storage with fixes.

Revert "Revert "Initial OpenGL Image support.""

This reverts commit 59dc41175d99d0a31c046aec0c26c4d82a3a3574.

BUG=skia:

Change-Id: Ibe3c87ce7f746f065fdbcc5a518388cc291112f5
Reviewed-on: https://skia-review.googlesource.com/5131
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
diff --git a/tools/gpu/GrTest.cpp b/tools/gpu/GrTest.cpp
index 8574399..14390ec 100644
--- a/tools/gpu/GrTest.cpp
+++ b/tools/gpu/GrTest.cpp
@@ -287,6 +287,8 @@
     explicit MockCaps(const GrContextOptions& options) : INHERITED(options) {}
     bool isConfigTexturable(GrPixelConfig config) const override { return false; }
     bool isConfigRenderable(GrPixelConfig config, bool withMSAA) const override { return false; }
+    bool canConfigBeImageStorage(GrPixelConfig) const override { return false; }
+
 private:
     typedef GrCaps INHERITED;
 };