Reenable support for UI Elements that use Render Targets (#2352)

* Re-add support for UI Elements that use Render Targets
* Move LyShine pass request from Atom's MainPipeline.pass to project's
* Make all dynamic draw contexts in LyShine draw to pass directly without the need of draw list tags
* Remove local RPI changes that are no longer needed
* Prevent crash if LyShine gem is enabled but its custom pass hasn't been added to the main render pipeline
* Revert to default UI pass if the LyShine pass has not been added to project's main render pipeline

Signed-off-by: abrmich <abrmich@amazon.com>

Gems/AtomLyIntegration/AtomBridge/Assets/Shaders/LyShineUI.azsl

@@ -10,9 +10,6 @@
 
 #include <Atom/Features/PostProcessing/PostProcessUtil.azsli>
 
-// Indicates whether to use pre-multiplied alpha
-option bool o_preMultiplyAlpha;
-
 // If true pixels with an alpha value of less than 0.5 are clipped
 option bool o_alphaTest;
 
@@ -86,9 +83,9 @@ struct PSOutput
     float4 m_color : SV_Target0;
 };
 
-float4 SampleTriangleTexture(int texIndex, float2 uv)
+float4 SampleTriangleTexture(uint texIndex, float2 uv)
 {
-    if ((InstanceSrg::m_isClamp & (1 << texIndex)) != 0)
+    if ((InstanceSrg::m_isClamp & (1U << texIndex)) != 0)
     {
         return InstanceSrg::m_texture[texIndex].Sample(InstanceSrg::m_clampSampler, uv);
     }
@@ -120,14 +117,6 @@ PSOutput MainPS(VSOutput IN)
         resColor.xyz = LinearToSRGB(resColor.xyz);
     }
 
-    // Check for flag to premultiply alpha
-    if (o_preMultiplyAlpha)
-    {
-        // premultiply the color by the alpha. This would not be required if we had full access to the separate alpha blend mode
-        float preMult = resColor.w;
-        resColor.xyz *= preMult;
-    }
-
     // If the o_modulate option is not None it means that the verts have two texture indicies. The second texture is used to
     // mask the first. This is used for gradient masks.
     if (o_modulate == Modulate::Alpha)

Gems/AtomLyIntegration/AtomBridge/Assets/Shaders/LyShineUI.shadervariantlist

@@ -4,7 +4,6 @@
         {
             "StableId": 1,
             "Options": {
-                "o_preMultiplyAlpha": "false",
                 "o_alphaTest": "false",
                 "o_srgbWrite": "true",
                 "o_modulate": "Modulate::None"
@@ -13,11 +12,26 @@
         {
             "StableId": 2,
             "Options": {
-                "o_preMultiplyAlpha": "false",
-                "o_alphaTest": "true",
-                "o_srgbWrite": "true",
+                "o_alphaTest": "false",
+                "o_srgbWrite": "false",
                 "o_modulate": "Modulate::None"
             }
+        },
+        {
+            "StableId": 3,
+            "Options": {
+                "o_alphaTest": "true",
+                "o_srgbWrite": "false",
+                "o_modulate": "Modulate::None"
+            }
+        },
+        {
+            "StableId": 4,
+            "Options": {
+                "o_alphaTest": "false",
+                "o_srgbWrite": "false",
+                "o_modulate": "Modulate::Alpha"
+            }
         }
     ]
 }