Added experimental.pixelShaderImagePath (#14073)

I realize I might be one of the few developers that care about custom
shader support in terminal but I thought it's worth proposing it and see
what you think.

This is to support custom shaders with custom textures.

I was thinking of exposing the background image to the shader but that
felt complicated after looking into it.

I have tested exploratively. I think the texture loader is possible to
unit test so that is a possible improvement.

The error reporting (as with other custom pixel shader code) is not very
good. That is also an area that I could improve upon.

I do think the risk of adding this is rather low as the new code is only
executed when experimental.pixelShaderImagePath is set.

### Details

Only added to the Atlas engine.

Instead I load the texture using WIC into a shader resource view. When
binding shader resources I test for presence of custom texture and bind
it to register t1.

The image loading code was found in [the D3D Texture documentation].
It's a mouthful but seems rather robust.


Tested setting: "experimental.pixelShaderImagePath"

1. Tested not specifying it.
2. Tested setting it.
3. Tested changing it (the changes are picked up)
4. Tested invalid path
5. Tested a custom shader that made use of the custom texture.

[the D3D Texture documentation]: https://learn.microsoft.com/en-us/windows/win32/direct3d11/overviews-direct3d-11-resources-textures-how-to

Co-authored-by: Mike Griese <migrie@microsoft.com>
Co-authored-by: Leonard Hecker <lhecker@microsoft.com>

samples/PixelShaders/BackgroundImage.hlsl

@@ -0,0 +1,23 @@
+// Demo shader to show passing in an image using
+// experimental.pixelShaderImagePath. This shader simply displays the Terminal
+// contents on top of the given image.
+//
+// The image loaded by the terminal will be placed into the `image` texture.
+
+SamplerState samplerState;
+Texture2D shaderTexture : register(t0);
+Texture2D image : register(t1);
+
+cbuffer PixelShaderSettings {
+    float Time;
+    float Scale;
+    float2 Resolution;
+    float4 Background;
+};
+
+float4 main(float4 pos : SV_POSITION, float2 tex : TEXCOORD) : SV_TARGET
+{
+    float4 terminalColor = shaderTexture.Sample(samplerState, tex);
+    float4 imageColor = image.Sample(samplerState, tex);
+    return lerp(imageColor, terminalColor, terminalColor.a);
+}