ES31: Implement multisampled Textures.

Implement TexStorage2DMultisample and getMultisamplefv entry point.
Also modify sample state for Textures and Framebuffers.

BUG=angleproject:1590
TEST=angle_unittests
TEST=angle_end2end_tests
TEST=dEQP-GLES31.functional.texture.multisample.samples_*.sample_position
TEST=dEQP-GLES31.functional.texture.multisample.samples_*.use_texture_color_2d
TEST=dEQP-GLES31.functional.texture.multisample.samples_*.use_texture_depth_2d
TEST=dEQP-GLES31.functional.texture.multisample.negative.fbo_attach_different_sample_count_tex_tex
TEST=dEQP-GLES31.functional.texture.multisample.negative.fbo_attach_different_sample_count_tex_rbo
TEST=dEQP-GLES31.functional.texture.multisample.negative.fbo_attach_non_zero_level
TEST=dEQP-GLES31.functional.texture.multisample.negative.texture_high_sample_count
TEST=dEQP-GLES31.functional.texture.multisample.negative.texture_zero_sample_count
TEST=dEQP-GLES31.functional.shaders.builtin_functions.texture_size.samples_1_texture_2d
TEST=dEQP-GLES31.functional.shaders.builtin_functions.texture_size.samples_4_texture_2d

Change-Id: I8fa7bd4e73b95745858a3e16b1b92004b4a18712
Reviewed-on: https://chromium-review.googlesource.com/414309
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Yunchao He <yunchao.he@intel.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
diff --git a/src/libANGLE/renderer/null/FramebufferNULL.cpp b/src/libANGLE/renderer/null/FramebufferNULL.cpp
index 91d3314..d0deef6 100644
--- a/src/libANGLE/renderer/null/FramebufferNULL.cpp
+++ b/src/libANGLE/renderer/null/FramebufferNULL.cpp
@@ -132,4 +132,9 @@
 {
 }
 
+gl::Error FramebufferNULL::getSamplePosition(size_t index, GLfloat *xy) const
+{
+    return gl::NoError();
+}
+
 }  // namespace rx