Enable sRGB on iOS, make sRGB decode support optional

tests/SRGBMipMapTest.cpp

 /*
  * Copyright 2016 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "Test.h"
 #if SK_SUPPORT_GPU
 #include "GrCaps.h"
 #include "GrContext.h"
 #include "GrRenderTargetContext.h"
+#include "gl/GrGLGpu.h"
 #include "SkCanvas.h"
 #include "SkSurface.h"
 
 // using anonymous namespace because these functions are used as template params.
 namespace {
 /** convert 0..1 srgb value to 0..1 linear */
 float srgb_to_linear(float srgb) {
     if (srgb <= 0.04045f) {
         return srgb / 12.92f;
     } else {
@@ skipping 112 matching lines @@
 
     // 1) Draw texture to S32 surface (should generate/use sRGB mips)
     paint.setGammaCorrect(true);
     s32RenderTargetContext->drawRect(noClip, paint, SkMatrix::I(), rect);
     read_and_check_pixels(reporter, s32RenderTargetContext->asTexture().get(), expectedSRGB, error,
                           "first render of sRGB");
 
     // 2) Draw texture to L32 surface (should generate/use linear mips)
     paint.setGammaCorrect(false);
     l32RenderTargetContext->drawRect(noClip, paint, SkMatrix::I(), rect);
-    read_and_check_pixels(reporter, l32RenderTargetContext->asTexture().get(), expectedLinear,
-                          error, "re-render as linear");
+
+    // Right now, this test only runs on GL (because Vulkan doesn't support legacy mip-mapping
+    // skbug.com/5048). On GL, we may not have sRGB decode support. In that case, rendering sRGB
+    // textures to a legacy surface produces nonsense, so this part of the test is meaningless.
+    //
+    // TODO: Once Vulkan supports legacy mip-mapping, we can promote this to GrCaps. Right now,
+    // Vulkan has most of the functionality, but not the mip-mapping part that's being tested here.
+    GrGLGpu* glGpu = static_cast<GrGLGpu*>(context->getGpu());
+    if (glGpu->glCaps().srgbDecodeDisableSupport()) {
+        read_and_check_pixels(reporter, l32RenderTargetContext->asTexture().get(), expectedLinear,
+                              error, "re-render as linear");
+    }
 
     // 3) Go back to sRGB
     paint.setGammaCorrect(true);
     s32RenderTargetContext->drawRect(noClip, paint, SkMatrix::I(), rect);
     read_and_check_pixels(reporter, s32RenderTargetContext->asTexture().get(), expectedSRGB, error,
                           "re-render as sRGB");
 }
 #endif