Add support for multiple shadowcaster planes

Extended ShadowcasterUpdater and shaders to handle multiple shadowcaster planes by introducing indexed properties and logic for two shadowcaster sets. Updated property names and shader logic to support per-index shadowcaster textures, colors, and brightness, enabling more flexible shadow casting in scenes.

Shader/Includes/Variables.hlsl

@@ -11,12 +11,4 @@
     float  _Udon_ShadowMapIndex[MAX_LIGHTS];\
     float  _Udon_PlayerCount;                  /* set via SetFloat */ \
 
-
-    float4x4 _Udon_WorldToLocal; \
-    sampler2D _shadowCasterTex; \
-    float4 _shadowCasterColor;  \
-    float4 _OutSideColor;       \
-    float _MinBrightnessShadow; \
-
-
 #endif