Fix for chromium GLSL compiler issue

[1chandu]

For #1112

Background

Empty Fragment shader used to work :

'void main() {}'

But when upgrading chromium fails all Transform related tests with this error :

[.WebGL-0x7fa0cf002a00]GL ERROR :GL_INVALID_OPERATION : glDrawArrays: buffer format and fragment output variable type incompatible

Change in this PR fixes the problem.

Change List

• Fix for chromium GLSL compiler issue

[1chandu]

@tsherif @Pessimistress
Could this be Chromium specific issue, I didn't find any in google search.

[coveralls]

Coverage remained the same at 45.063% when pulling 516676a on TransformError into c5dc527 on master.

[tsherif]

It seems that behavior is correct for WebGL 2:

If any draw buffer with an attachment does not have a defined fragment shader output, draws generate INVALID_OPERATION, unless all 4 channels of colorMask are set to false.

Odd that there's inconsistency across browsers though...

[tsherif]

It was a recent update to the spec: KhronosGroup/WebGL#2780

[Pessimistress]

I first observed this issue in Chromium 76. Is this going to be an issue for everyone when Chrome updates? (i.e. do we need to patch 6.x)?

[tsherif]

I imagine it won't be an issue unless there's code somewhere that uses GLSL 3 shaders that don't output anything?

[1chandu]

It was a recent update to the spec: KhronosGroup/WebGL#2780

Thanks for the link.
For all TF paths we also set RASTERIZER_DISCARD to true but wonder thats not enough. Will merge this PR given this should be safe change.

[tsherif]

If RASTERIZER_DISCARD isn't working, I'd say that's a bug. We should file it with Chromium: https://bugs.chromium.org/p/chromium/issues/list