Always declare sk_FragColor in GLSL, even if unused

PLS and discard-only shaders are the only time this has an impact,
and it doesn't seem like a problem to have the declaration?

Removes one use of variable reference counts, which are going to be
refactored.

Change-Id: Idb8d06087eed56070252ee02dcf907bf0d24c5a1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/328796
Reviewed-by: John Stiles <johnstiles@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>

Author: brianosman

src/sksl/SkSLGLSLCodeGenerator.cpp

@@ -1495,8 +1495,7 @@ void GLSLCodeGenerator::writeProgramElement(const ProgramElement& e) {
                 this->writeVarDeclaration(decl, true);
                 this->writeLine();
             } else if (builtin == SK_FRAGCOLOR_BUILTIN &&
-                       fProgram.fSettings.fCaps->mustDeclareFragmentShaderOutput() &&
-                       decl.var().writeCount()) {
+                       fProgram.fSettings.fCaps->mustDeclareFragmentShaderOutput()) {
                 if (fProgram.fSettings.fFragColorIsInOut) {
                     this->write("inout ");
                 } else {

tests/sksl/shared/golden/Discard.glsl

@@ -1,4 +1,5 @@
 
+out vec4 sk_FragColor;
 void main() {
     {
         discard;