internal/shader: more strict type check for the built-in cast-like functions

[hajimehoshi]

Ebitengine Version

ef08873

Operating System

• Windows
• macOS
• Linux
• FreeBSD
• OpenBSD
• Android
• iOS
• Nintendo Switch
• Xbox
• Web Browsers

Go Version (go version)

go version go1.20.4 darwin/amd64

What steps will reproduce the problem?

Try to compile this Kage program

package main

func Foo() {
        _ = ivec2(3.1, 4.1)
}

func Fragment(position vec4, texCoord vec2, color vec4) vec4 {
        return vec4(1, 0, 0, 1)
}

What is the expected result?

Fails to comiple due to an inconsistent types

What happens instead?

Succeeds to compile.

Anything else you feel useful to add?

The same issue is in any other built-in functions.

This is a kind of breaking change. I am not sure we should fix this in v2.6. I'll set the milestone v2.7.

[hajimehoshi]

I'll fix this for ivecN now as there should be few users.