Fixed visual bugs with brushes and fast mouse clicks

Reproducable with mouse keys (used incorrectly) to produce two clicks in rapid succession
Causes the rectangle tool to first place the preview then a 1x1 at the start
The 1x1 used 0 as fallback for missing data instead of " ", resulting in it "deleting" the visuals

src/brushes.lua

@@ -126,7 +126,7 @@ function brushHelper.updateRender(room, x, y, material, layer, randomMatrix)
                 local tx, ty = x + i - 1, y + j - 1
 
                 if tx >= 1 and ty >= 1 and tx <= tilesWidth and ty <= tilesHeight then
-                    local target = tilesMatrix:get(tx, ty, "0")
+                    local target = tilesMatrix:get(tx, ty, " ")
                     local mat = material:getInbounds(i, j)
 
                     if mat ~= target and mat ~= " " then