WebGL adds two rules:
 - Fragment outputs declared but not written to should default to black.
 - FBO attachments for outputs not declared in the shader should not be
written to (it is UB in OpenGL ES).

Fix the first one by using the SH_INIT_OUTPUT_VARIABLES compiler
options, and the second one by messing with glDrawBuffers so that the
enabled draw buffers are always a subset of the ones declared by the
shader.

BUG=angleproject:2048

Change-Id: I1d851c190959c1acfc3e41d837e6990aec1d4086
Reviewed-on: https://chromium-review.googlesource.com/521682
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>