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/samples/compute_boids.wgsl.expected.glsl

@@ -29,6 +29,7 @@ vec4 vert_main(vec2 a_particlePos, vec2 a_particleVel, vec2 a_pos) {
 }
 
 void main() {
+  gl_PointSize = 1.0;
   vec4 inner_result = vert_main(a_particlePos_1, a_particleVel_1, a_pos_1);
   gl_Position = inner_result;
   gl_Position.y = -(gl_Position.y);

test/tint/samples/cube.wgsl.expected.glsl

@@ -29,6 +29,7 @@ VertexOutput vtx_main(VertexInput tint_symbol) {
 }
 
 void main() {
+  gl_PointSize = 1.0;
   VertexInput tint_symbol_2 = VertexInput(cur_position_1, color_1);
   VertexOutput inner_result = vtx_main(tint_symbol_2);
   vtxFragColor_1 = inner_result.vtxFragColor;

test/tint/samples/simple_vertex.spvasm.expected.glsl

@@ -17,6 +17,7 @@ main_out tint_symbol_1() {
 }
 
 void main() {
+  gl_PointSize = 1.0;
   main_out inner_result = tint_symbol_1();
   gl_Position = inner_result.tint_symbol;
   gl_Position.y = -(gl_Position.y);

test/tint/samples/triangle.wgsl.expected.glsl

@@ -6,6 +6,7 @@ vec4 vtx_main(uint VertexIndex) {
 }
 
 void main() {
+  gl_PointSize = 1.0;
   vec4 inner_result = vtx_main(uint(gl_VertexID));
   gl_Position = inner_result;
   gl_Position.y = -(gl_Position.y);