Fix y billboard shear when rotating camera

Fixes shear effect with `BILLBOARD_FIXED_Y` when the camera is rotated around the z-axis by rotating the mesh correctly into view space.

Also removes shearing effects that occur when rotating the mesh by excluding the model rotation and scale from the billboard matrix.

(cherry picked from commit d3850a0954e472e47aa1ce0bd946e6db0fbb309e)

scene/resources/material.cpp

@@ -647,12 +647,10 @@ void SpatialMaterial::_update_shader() {
 			}
 		} break;
 		case BILLBOARD_FIXED_Y: {
-			code += "\tMODELVIEW_MATRIX = INV_CAMERA_MATRIX * mat4(CAMERA_MATRIX[0],WORLD_MATRIX[1],vec4(normalize(cross(CAMERA_MATRIX[0].xyz,WORLD_MATRIX[1].xyz)), 0.0),WORLD_MATRIX[3]);\n";
+			code += "\tMODELVIEW_MATRIX = INV_CAMERA_MATRIX * mat4(vec4(normalize(cross(vec3(0.0, 1.0, 0.0), CAMERA_MATRIX[2].xyz)),0.0),vec4(0.0, 1.0, 0.0, 0.0),vec4(normalize(cross(CAMERA_MATRIX[0].xyz, vec3(0.0, 1.0, 0.0))),0.0),WORLD_MATRIX[3]);\n";
 
 			if (flags[FLAG_BILLBOARD_KEEP_SCALE]) {
-				code += "\tMODELVIEW_MATRIX = MODELVIEW_MATRIX * mat4(vec4(length(WORLD_MATRIX[0].xyz), 0.0, 0.0, 0.0),vec4(0.0, 1.0, 0.0, 0.0),vec4(0.0, 0.0, length(WORLD_MATRIX[2].xyz), 0.0), vec4(0.0, 0.0, 0.0, 1.0));\n";
-			} else {
-				code += "\tMODELVIEW_MATRIX = MODELVIEW_MATRIX * mat4(vec4(1.0, 0.0, 0.0, 0.0),vec4(0.0, 1.0/length(WORLD_MATRIX[1].xyz), 0.0, 0.0), vec4(0.0, 0.0, 1.0, 0.0),vec4(0.0, 0.0, 0.0 ,1.0));\n";
+				code += "\tMODELVIEW_MATRIX = MODELVIEW_MATRIX * mat4(vec4(length(WORLD_MATRIX[0].xyz), 0.0, 0.0, 0.0),vec4(0.0, length(WORLD_MATRIX[1].xyz), 0.0, 0.0),vec4(0.0, 0.0, length(WORLD_MATRIX[2].xyz), 0.0),vec4(0.0, 0.0, 0.0, 1.0));\n";
 			}
 		} break;
 		case BILLBOARD_PARTICLES: {