Add support for half float alpha textures.

src/gpu/gl/GrGpuGL.cpp

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 
 #include "GrGpuGL.h"
 #include "GrGLStencilBuffer.h"
@@ skipping 2227 matching lines @@
         case kASTC_12x12_GrPixelConfig:
             *internalFormat = GR_GL_COMPRESSED_RGBA_ASTC_12x12;
             break;
 
         case kRGBA_float_GrPixelConfig:
             *internalFormat = GR_GL_RGBA32F;
             *externalFormat = GR_GL_RGBA;
             *externalType = GR_GL_FLOAT;
             break;
 
+        case kAlpha_half_GrPixelConfig:
+            if (this->glCaps().textureRedSupport()) {
+                *internalFormat = GR_GL_R16F;
+                *externalFormat = GR_GL_RED;
+                *externalType = (kGLES_GrGLStandard == this->glStandard()) ? GR_GL_HALF_FLOAT
+                                                                           : GR_GL_HALF_FLOAT_OES;
+            } else {
+                *internalFormat = GR_GL_ALPHA16F;
+                *externalFormat = GR_GL_ALPHA;
+                *externalType = (kGLES_GrGLStandard == this->glStandard()) ? GR_GL_HALF_FLOAT
+                                                                           : GR_GL_HALF_FLOAT_OES;
+            }
+            break;
+            
         default:
             return false;
     }
     return true;
 }
 
 void GrGpuGL::setTextureUnit(int unit) {
     SkASSERT(unit >= 0 && unit < fHWBoundTextureUniqueIDs.count());
     if (unit != fHWActiveTextureUnitIdx) {
         GL_CALL(ActiveTexture(GR_GL_TEXTURE0 + unit));
@@ skipping 315 matching lines @@
             this->setVertexArrayID(gpu, 0);
         }
         int attrCount = gpu->glCaps().maxVertexAttributes();
         if (fDefaultVertexArrayAttribState.count() != attrCount) {
             fDefaultVertexArrayAttribState.resize(attrCount);
         }
         attribState = &fDefaultVertexArrayAttribState;
     }
     return attribState;
 }