Add EGL_TEXTURE_OFFSET_X_ANGLE and EGL_TEXTURE_OFFSET_Y_ANGLE attributes
to EGL_ANGLE_d3d_texture_client_buffer which are used to specify offsets
used for rendering into the pbuffer surface backed by a D3D11 texture.
Rendering with the correct offset is needed when Chrome gets a texture
and offset from an external source e.g. when calling BeginDraw() on
IDCompositionSurface.

Bug: angleproject:2997
Change-Id: I363f739f3f05c38720f385e34c91e98fc6a622a0
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2186176Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: Sunny Sachanandani <sunnyps@chromium.org>