Allow emit_subparticle from start function

emit_subparticle was previously only allowed from the process function
in a particle shader, which seemed like an unnecessary limitation.

servers/rendering/shader_types.cpp

@@ -340,6 +340,11 @@ ShaderTypes::ShaderTypes() {
 	shader_modes[RS::SHADER_PARTICLES].functions["start"].built_ins["INDEX"] = constt(ShaderLanguage::TYPE_UINT);
 	shader_modes[RS::SHADER_PARTICLES].functions["start"].built_ins["EMISSION_TRANSFORM"] = constt(ShaderLanguage::TYPE_MAT4);
 	shader_modes[RS::SHADER_PARTICLES].functions["start"].built_ins["RANDOM_SEED"] = constt(ShaderLanguage::TYPE_UINT);
+	shader_modes[RS::SHADER_PARTICLES].functions["start"].built_ins["FLAG_EMIT_POSITION"] = constt(ShaderLanguage::TYPE_UINT);
+	shader_modes[RS::SHADER_PARTICLES].functions["start"].built_ins["FLAG_EMIT_ROT_SCALE"] = constt(ShaderLanguage::TYPE_UINT);
+	shader_modes[RS::SHADER_PARTICLES].functions["start"].built_ins["FLAG_EMIT_VELOCITY"] = constt(ShaderLanguage::TYPE_UINT);
+	shader_modes[RS::SHADER_PARTICLES].functions["start"].built_ins["FLAG_EMIT_COLOR"] = constt(ShaderLanguage::TYPE_UINT);
+	shader_modes[RS::SHADER_PARTICLES].functions["start"].built_ins["FLAG_EMIT_CUSTOM"] = constt(ShaderLanguage::TYPE_UINT);
 	shader_modes[RS::SHADER_PARTICLES].functions["start"].built_ins["RESTART_POSITION"] = constt(ShaderLanguage::TYPE_BOOL);
 	shader_modes[RS::SHADER_PARTICLES].functions["start"].built_ins["RESTART_ROT_SCALE"] = constt(ShaderLanguage::TYPE_BOOL);
 	shader_modes[RS::SHADER_PARTICLES].functions["start"].built_ins["RESTART_VELOCITY"] = constt(ShaderLanguage::TYPE_BOOL);
@@ -385,6 +390,7 @@ ShaderTypes::ShaderTypes() {
 		emit_vertex_func.arguments.push_back(ShaderLanguage::StageFunctionInfo::Argument("custom", ShaderLanguage::TYPE_VEC4));
 		emit_vertex_func.arguments.push_back(ShaderLanguage::StageFunctionInfo::Argument("flags", ShaderLanguage::TYPE_UINT));
 		emit_vertex_func.return_type = ShaderLanguage::TYPE_BOOL; //whether it could emit
+		shader_modes[RS::SHADER_PARTICLES].functions["start"].stage_functions["emit_subparticle"] = emit_vertex_func;
 		shader_modes[RS::SHADER_PARTICLES].functions["process"].stage_functions["emit_subparticle"] = emit_vertex_func;
 	}