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>

test/tint/builtins/textureLoad/depth_ms.spvasm.expected.glsl

@@ -31,6 +31,7 @@ vertex_main_out vertex_main() {
 }
 
 void main() {
+  gl_PointSize = 1.0;
   vertex_main_out inner_result = vertex_main();
   gl_Position = inner_result.tint_symbol_1_1;
   gl_Position.y = -(gl_Position.y);

test/tint/builtins/textureLoad/texture_external_param.wgsl.expected.glsl

@@ -48,6 +48,7 @@ vec4 vertex_main() {
 }
 
 void main() {
+  gl_PointSize = 1.0;
   vec4 inner_result = vertex_main();
   gl_Position = inner_result;
   gl_Position.y = -(gl_Position.y);

test/tint/builtins/textureLoad/texture_param.wgsl.expected.glsl

@@ -15,6 +15,7 @@ vec4 vertex_main() {
 }
 
 void main() {
+  gl_PointSize = 1.0;
   vec4 inner_result = vertex_main();
   gl_Position = inner_result;
   gl_Position.y = -(gl_Position.y);