Enable sRGB on iOS, make sRGB decode support optional

src/gpu/gl/GrGLGpu.cpp

Index: src/gpu/gl/GrGLGpu.cpp
diff --git a/src/gpu/gl/GrGLGpu.cpp b/src/gpu/gl/GrGLGpu.cpp
index 67414a495789da2dd21381db670bdc0073eb0692..8cec2c910855afe3287a0a8881b940487234bcec 100644
--- a/src/gpu/gl/GrGLGpu.cpp
+++ b/src/gpu/gl/GrGLGpu.cpp
@@ -3244,7 +3244,7 @@ void GrGLGpu::bindTexture(int unitIdx, const GrSamplerParams& params, bool allow
     newTexParams.fMinFilter = glMinFilterModes[filterMode];
     newTexParams.fMagFilter = glMagFilterModes[filterMode];
 
-    if (GrPixelConfigIsSRGB(texture->config())) {
+    if (this->glCaps().srgbDecodeSupport() && GrPixelConfigIsSRGB(texture->config())) {
         newTexParams.fSRGBDecode = allowSRGBInputs ? GR_GL_DECODE_EXT : GR_GL_SKIP_DECODE_EXT;
         if (setAll || newTexParams.fSRGBDecode != oldTexParams.fSRGBDecode) {
             this->setTextureUnit(unitIdx);
@@ -3420,7 +3420,7 @@ void GrGLGpu::generateMipmaps(const GrSamplerParams& params, bool allowSRGBInput
 
     // Configure sRGB decode, if necessary. This state is the only thing needed for the driver
     // call (glGenerateMipmap) to work correctly. Our manual method dirties other state, too.
-    if (GrPixelConfigIsSRGB(texture->config())) {
+    if (this->glCaps().srgbDecodeSupport() && GrPixelConfigIsSRGB(texture->config())) {
         GL_CALL(TexParameteri(target, GR_GL_TEXTURE_SRGB_DECODE_EXT,
                               allowSRGBInputs ? GR_GL_DECODE_EXT : GR_GL_SKIP_DECODE_EXT));
     }