BUG=angleproject:1002

A 'LoadFunction' is used to load data of a given <GL format, GL type> pair into
a specific DXGI_FORMAT. Before this change, BuildD3D11LoadFunctionMap mapped
<GL format, GL type> pairs to LoadFunctions. This meant each
<GL format, GL type> pair could only loaded using one LoadFunction, and
therefore could only be loaded into one specific DXGI_FORMAT.

We now want to map <GL format, GL type, DXGI_FORMAT> triples to LoadFunctions.
This allows us to use different DXGI_FORMATS for each <GL format, GL type> pair,
depending on the capabilities of the underlying D3D11 device.

Change-Id: I3b5e24c4bf98f18360ddd45b2022a90b5000f866
Reviewed-on: https://chromium-review.googlesource.com/270561Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Austin Kinross <aukinros@microsoft.com>