Text widget: intersect scissor.

Fixes a visual bug where text would display outside widget boundary.
Mved intersectScissor to Backend.

luigi/backend.lua

@@ -1,8 +1,27 @@
 local ROOT = (...):gsub('[^.]*$', '')
+local Backend
 
 if _G.love and _G.love._version_minor > 8 then
-    return require(ROOT .. 'backend.love')
+    Backend = require(ROOT .. 'backend.love')
 else
-    return require(ROOT .. 'backend.ffisdl')
+    Backend = require(ROOT .. 'backend.ffisdl')
 end
 
+Backend.intersectScissor = Backend.intersectScissor or function (x, y, w, h)
+    local sx, sy, sw, sh = Backend.getScissor()
+    if not sx then
+        return Backend.setScissor(x, y, w, h)
+    end
+    local x1 = math.max(sx, x)
+    local y1 = math.max(sy, y)
+    local x2 = math.min(sx + sw, x + w)
+    local y2 = math.min(sy + sh, y + h)
+    if x2 > x1 and y2 > y1 then
+        Backend.setScissor(x1, y1, x2 - x1, y2 - y1)
+    else
+        -- HACK
+        Backend.setScissor(-100, -100, 1, 1)
+    end
+end
+
+return Backend