optimize precision for conversion from srgb to linear

Convertion from sRGB to 8-bit linear is going to reduce the
precision of the texture, and the downsampling will do the
same thing. We should set texture internalformat to rgba32f
to optimize precision.

BUG=634519
TEST=conformance2/textures/misc/tex-srgb-mipmap.html
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2420813002
Cr-Commit-Position: refs/heads/master@{#427014}

gpu/command_buffer/service/feature_info.h

@@ -161,6 +161,14 @@ class GPU_EXPORT FeatureInfo : public base::RefCounted<FeatureInfo> {
   void EnableOESTextureFloatLinear();
   void EnableOESTextureHalfFloatLinear();
 
+  bool ext_color_buffer_float_available() const {
+    return ext_color_buffer_float_available_;
+  }
+
+  bool oes_texture_float_linear_available() const {
+    return oes_texture_float_linear_available_;
+  }
+
  private:
   friend class base::RefCounted<FeatureInfo>;
   friend class BufferManagerClientSideArraysTest;

gpu/command_buffer/service/gles2_cmd_srgb_converter.cc

@@ -360,10 +360,15 @@ void SRGBConverter::GenerateMipmap(const gles2::GLES2Decoder* decoder,
   const GLint mipmap_levels =
       TextureManager::ComputeMipMapCount(target, width, height, depth);
 
-  // bind srgb_decoder_textures_[1] to draw framebuffer
   glBindTexture(GL_TEXTURE_2D, srgb_converter_textures_[1]);
-  glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, GL_RGBA,
+  if (feature_info_->ext_color_buffer_float_available() &&
+      feature_info_->oes_texture_float_linear_available()) {
+    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA32F, width, height, 0, GL_RGBA,
                  GL_UNSIGNED_BYTE, nullptr);
+  } else {
+    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8, width, height, 0, GL_RGBA,
+                 GL_UNSIGNED_BYTE, nullptr);
+  }
   glBindFramebufferEXT(GL_DRAW_FRAMEBUFFER, srgb_decoder_fbo_);
   glFramebufferTexture2DEXT(GL_DRAW_FRAMEBUFFER, GL_COLOR_ATTACHMENT0,
                             GL_TEXTURE_2D, srgb_converter_textures_[1], 0);
@@ -387,7 +392,6 @@ void SRGBConverter::GenerateMipmap(const gles2::GLES2Decoder* decoder,
 
   glDrawArrays(GL_TRIANGLES, 0, 6);
 
-  // generateMipmap for srgb_decoder_textures_[1]
   glBindTexture(GL_TEXTURE_2D, srgb_converter_textures_[1]);
   glGenerateMipmapEXT(GL_TEXTURE_2D);