Implement GL support for buffer textures

src/gpu/gl/GrGLProgramDesc.cpp

Index: src/gpu/gl/GrGLProgramDesc.cpp
diff --git a/src/gpu/gl/GrGLProgramDesc.cpp b/src/gpu/gl/GrGLProgramDesc.cpp
index 68020a92d5977accaa513eabb52944a6ffffdec9..1222648dd15c6c4fbc7a60f5dddfef74e0ae6a4e 100644
--- a/src/gpu/gl/GrGLProgramDesc.cpp
+++ b/src/gpu/gl/GrGLProgramDesc.cpp
@@ -25,6 +25,8 @@ static uint8_t texture_target_key(GrGLenum target) {
             return 1;
         case GR_GL_TEXTURE_RECTANGLE:
             return 2;
+        case GR_GL_TEXTURE_BUFFER:
+            return 3;
         default:
             SkFAIL("Unexpected texture target.");
             return 0;