[glsl] Inject default gl_PointSize = 1.0 in vertex shaders if FORCE_POINT_SIZE option was set

[REASY]

Write gl_PointSize = 1.0; in case of vertex shader and set writer flag FORCE_POINT_SIZE

According to https://registry.khronos.org/OpenGL/specs/es/3.2/GLSL_ES_Specification_3.20.html#built-in-language-variables

The variable gl_PointSize is intended for a shader to write the size of the point to be rasterized. It is measured in pixels. If gl_PointSize is not written to, its value is undefined in subsequent pipe stages.

This is the first part of the changes to address gfx-rs/wgpu#3179

[teoxoy]

I think this needs to be implemented in the same way it is for the spv backend.

naga/src/back/spv/writer.rs

Lines 524 to 526 in c7d0215

	if self.flags.contains(WriterFlags::FORCE_POINT_SIZE)
	&& iface.stage == crate::ShaderStage::Vertex
	&& !has_point_size

• rename allow_point_size to force_point_size
• check if we already have (and wrote/will write) the BuiltIn::PointSize

[JCapucho]

It would also help to include a comment in the code explaining this for future reference with a mention of the section of where it happens in the specification (for glsl ES 3.1 this is in section 7.1.1)

[teoxoy]

LGTM, thanks!