Exposes gl_SecondaryFragColor, glSecondaryFragData[] and
gl_MaxDualSourceDrawBuffers to GLES SL 1.0.

Relaxes rules for undefined output locations for GLES SL 3.0
and exposes gl_MaxDualSourceDrawBuffers.

If the output GL context is GL ES 2.0 or 3.0:
The emulation layer is expected to turn on EXT_blend_func_extended
if the output GL context supports it.

If the output GL context is GL:
The emulation layer is expected to turn on EXT_blend_func_extended
if the output GL context supports ARB_blend_func_extended or if GL
context is 3.2 or later.

If the source shader spec is GLES SL 2.0: The emulation layer is
expected to inspect the shader compilation output variables upon
linking. If output target is GL SL, the emulation layer should bind
color location 0, index 1 to "angle_SecondaryFragColor" if variable
"gl_SecondaryFragColorEXT" is used. Alternatively, emulation layer
should bind "angle_SecondaryFragData" to locations 0,1,2,3,..., all
color index 1, if "gl_SecondaryFragData" array is used.
(The caller can not bind the locations or specify output variables.)

If the source shader spec is GLES SL 3.0:
The emulation layer is expected to do location auto-resolve of the
the output variables that have undefined output locations that have
not been bound by the caller.
(The caller can not use gl_ built-ins, so nothing to do with those.)

BUG=angleproject:1085
TEST=angle_unittest

Change-Id: I5cafe205b0c29478b0dcd24aa89a7b0000f5d046
Reviewed-on: https://chromium-review.googlesource.com/287580Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Kimmo Kinnunen <kkinnunen@nvidia.com>