ES31: Implement FramebufferTextureEXT entry point

This patch adds the entry point and related validation for
FramebufferTextureEXT defined in OpenGL ES 3.1 extension
EXT_geometry_shader.

BUG=angleproject:1941
TEST=angle_end2end_tests

Change-Id: Id6804e0b3971f52273562ce1a325d8377926a558
Reviewed-on: https://chromium-review.googlesource.com/1069842
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
diff --git a/src/libANGLE/Context.cpp b/src/libANGLE/Context.cpp
index e38b85d..052d91a 100644
--- a/src/libANGLE/Context.cpp
+++ b/src/libANGLE/Context.cpp
@@ -3688,6 +3688,12 @@
     mGLState.setObjectDirty(target);
 }
 
+// TODO(jiawei.shao@intel.com): implement framebufferTextureEXT
+void Context::framebufferTexture(GLenum target, GLenum attachment, GLuint texture, GLint level)
+{
+    UNIMPLEMENTED();
+}
+
 void Context::drawBuffers(GLsizei n, const GLenum *bufs)
 {
     Framebuffer *framebuffer = mGLState.getDrawFramebuffer();