Handle single-argument constructors in shaders.

Expand these (fill vectors, fill matrix diagonals) in _reduce_expression() so they're handled properly even as uniform default values (where they previously caused a crash).

servers/visual/shader_language.cpp

@@ -3100,6 +3100,60 @@ ShaderLanguage::Node *ShaderLanguage::_reduce_expression(BlockNode *p_block, Sha
 			}
 		}
 
+		// Passed a single argument
+		if (op->arguments.size() <= 2) {
+			if (op->get_datatype() < TYPE_MAT2) {
+				// Scalar or vector
+				int dimensions = 0;
+				switch (op->get_datatype()) {
+					case TYPE_VEC2:
+					case TYPE_BVEC2:
+					case TYPE_UVEC2:
+					case TYPE_IVEC2: {
+						dimensions = 2;
+					} break;
+					case TYPE_VEC3:
+					case TYPE_BVEC3:
+					case TYPE_UVEC3:
+					case TYPE_IVEC3: {
+						dimensions = 3;
+					} break;
+					case TYPE_VEC4:
+					case TYPE_BVEC4:
+					case TYPE_UVEC4:
+					case TYPE_IVEC4: {
+						dimensions = 4;
+					} break;
+				}
+
+				if (dimensions) {
+					ConstantNode::Value val = values[0];
+					for (int i = 1; i < dimensions; i++) {
+
+						values.push_back(val);
+					}
+				}
+			} else if (op->get_datatype() <= TYPE_MAT4) {
+				// Matrix
+				int dimensions = 0;
+				switch (op->get_datatype()) {
+					case TYPE_MAT2: dimensions = 2; break;
+					case TYPE_MAT3: dimensions = 3; break;
+					case TYPE_MAT4: dimensions = 4; break;
+				}
+
+				// Fill the diagonal with the value, the rest with zeroes
+				ConstantNode::Value val = values[0];
+				for (int i = 1; i < dimensions * dimensions; i++) {
+					if (i % (dimensions + 1) == 0) {
+						values.push_back(val);
+					} else {
+						values.push_back(ConstantNode::Value());
+					}
+				}
+			}
+		}
+
 		ConstantNode *cn = alloc_node<ConstantNode>();
 		cn->datatype = op->get_datatype();
 		cn->values = values;