GLSL writer: emit gl_PointSize = 1 in all vertex shaders for OpenGL.

The OpenGL ES 3.1 spec says:

"The point size is taken from the shader built-in gl_PointSize written by the vertex shader, and clamped to the implementation-dependent point size range. If the value written to gl_PointSize is less than or equal to zero, or if no value is written, the point size is undefined."

ANGLE/D3D11 will emit a warning on this and abort processing, causing dawn_end2end tests to fail.

Bug: dawn:1390
Change-Id: I6a56467842ba80d8a070af7240ee07bb111089dd
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/87720
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Stephen White <senorblanco@chromium.org>

src/tint/writer/glsl/generator_impl.cc

@@ -2207,6 +2207,9 @@ bool GeneratorImpl::EmitEntryPointFunction(const ast::Function* func) {
   // Emit original entry point function body
   {
     ScopedIndent si(this);
+    if (func->PipelineStage() == ast::PipelineStage::kVertex) {
+      line() << "gl_PointSize = 1.0;";
+    }
 
     if (!EmitStatements(func->body->statements)) {
       return false;

src/tint/writer/glsl/generator_impl_function_test.cc

@@ -234,6 +234,7 @@ Interface vert_main() {
 }
 
 void main() {
+  gl_PointSize = 1.0;
   Interface inner_result = vert_main();
   gl_Position = inner_result.pos;
   col1_1 = inner_result.col1;