Add shadow caster support to LitParticles shader

Introduced shadow caster texture, color, outside color, and minimum brightness parameters to the LitParticles shader. Moved related variable declarations to Variables.hlsl for reuse and removed redundant declarations from BlendinShader.shader. Updated LitParticles lighting calculation to apply shadow caster effects when appropriate.

Shader/Includes/Variables.hlsl

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