internal/shaderir: add an option to choose `texels` or `pixels` to Kage shaders

[hajimehoshi]

The difficult part is how to keep the efficiency of the shaders.

[hajimehoshi]

In OpenGL, texelFecth is available: https://registry.khronos.org/OpenGL-Refpages/es3.0/html/texelFetch.xhtml, https://registry.khronos.org/OpenGL-Refpages/gl4/html/texelFetch.xhtml (For WebGL, WebGL 2 is requried)
In Metal, read is available instead of sample: https://developer.apple.com/metal/Metal-Shading-Language-Specification.pdf
In DirectX, Load is available instead of Sample: https://learn.microsoft.com/en-us/windows/win32/direct3dhlsl/dx-graphics-hlsl-to-load

[hajimehoshi]

Oops, as this requires WebGL2, I cannot add this immediately.

[hajimehoshi]

#2191 iOS/Safari 15 was released recently. Let's wait more, though iOS 14 is already discontinued.

[hajimehoshi]

#1911 is a blocker.