reimplement/fix dimRect() and lightenRect()

dimRect() and lightenRect() did assume that 0=white, 15=black
(old 4bpp inverted colors), thus did work the other way around.
Fix this. Drop the pixel dim/lighten methods, rather use a
blend with WHITE and (by default 50%) alpha.

The lightenRect() method changed in its inner workings by this
patch. Before, it applied a minimum value, leaving bigger
values untouched. Now, it is blending, i.e. making an alpha
blend with black (yes, black, not white, see below).
That reduces code footprint, and may look slightly better, too.

TODO: rename this methods. They do exactly the opposite as what
their names suggest.

ffi/blitbuffer.lua

@@ -202,33 +202,6 @@ function ColorRGB32_mt.__index:blend(color)
     self:set(ColorRGB32(r, g, b, self:getAlpha()))
 end
 
--- dimming
-function Color4L_mt.__index:dim()
-    return Color8(rshift(self:getColor8().a, 1))
-end
-Color4U_mt.__index.dim = Color4L_mt.__index.dim
-Color8_mt.__index.dim = Color4L_mt.__index.dim
-Color8A_mt.__index.dim = Color4L_mt.__index.dim
-ColorRGB16_mt.__index.dim = Color4L_mt.__index.dim
-ColorRGB24_mt.__index.dim = Color4L_mt.__index.dim
-ColorRGB32_mt.__index.dim = Color4L_mt.__index.dim
--- lighten up
-function Color4L_mt.__index:lighten(low)
-    local value = self:getColor4L().a
-    low = low * 0x0F
-    if value < low then
-        return Color4L(low)
-    else
-        return self
-    end
-end
-Color4U_mt.__index.lighten = Color4L_mt.__index.lighten
-Color8_mt.__index.lighten = Color4L_mt.__index.lighten
-Color8A_mt.__index.lighten = Color4L_mt.__index.lighten
-ColorRGB16_mt.__index.lighten = Color4L_mt.__index.lighten
-ColorRGB24_mt.__index.lighten = Color4L_mt.__index.lighten
-ColorRGB32_mt.__index.lighten = Color4L_mt.__index.lighten
-
 -- color conversions:
 -- to Color4L:
 function Color4L_mt.__index:getColor4L() return Color4L(band(0x0F, self.a)) end
@@ -861,15 +834,17 @@ paint a rectangle onto this buffer
 @param w width
 @param h height
 @param value color value
+@param setter function used to set pixels (defaults to normal setPixel)
 --]]
-function BB_mt.__index:paintRect(x, y, w, h, value)
+function BB_mt.__index:paintRect(x, y, w, h, value, setter)
+    setter = setter or self.setPixel
     value = value or Color8(0)
     if w <= 0 or h <= 0 then return end
     w, x = BB.checkBounds(w, x, 0, self:getWidth(), 0xFFFF)
     h, y = BB.checkBounds(h, y, 0, self:getHeight(), 0xFFFF)
     for y = y, y+h-1 do
         for x = x, x+w-1 do
-            self:setPixel(x, y, value)
+            setter(self, x, y, value)
         end
     end
 end
@@ -1089,24 +1064,19 @@ function BB_mt.__index:progressBar(x, y, w, h, load_m_w, load_m_h, load_percent,
     (w-2*load_m_w)*load_percent, (h-2*load_m_h), c)
 end
 
-
 --[[
 dim color values in rectangular area
 
 @param x X coordinate
 @param y Y coordinate
 @param w width
 @param h height
+@param by dim by this factor (default: 0.5)
 --]]
-function BB_mt.__index:dimRect(x, y, w, h)
-    if w <= 0 or h <= 0 then return end
-    w, x = BB.checkBounds(w, x, 0, self:getWidth(), 0xFFFF)
-    h, y = BB.checkBounds(h, y, 0, self:getHeight(), 0xFFFF)
-    for y = y, y+h-1 do
-        for x = x, x+w-1 do
-            self:setPixel(x, y, self:getPixel(x, y):dim())
-        end
-    end
+function BB_mt.__index:dimRect(x, y, w, h, by)
+    return self:paintRect(x, y, w, h,
+        Color8A(255, 255*(by or 0.5)),
+        self.setPixelBlend)
 end
 
 --[[
@@ -1116,16 +1086,12 @@ lighten color values in rectangular area
 @param y Y coordinate
 @param w width
 @param h height
+@param by lighten by this factor (default: 0.5)
 --]]
-function BB_mt.__index:lightenRect(x, y, w, h, low)
-    if w <= 0 or h <= 0 then return end
-    w, x = BB.checkBounds(w, x, 0, self:getWidth(), 0xFFFF)
-    h, y = BB.checkBounds(h, y, 0, self:getHeight(), 0xFFFF)
-    for y = y, y+h-1 do
-        for x = x, x+w-1 do
-            self:setPixel(x, y, self:getPixel(x, y):lighten(low))
-        end
-    end
+function BB_mt.__index:lightenRect(x, y, w, h, by)
+    return self:paintRect(x, y, w, h,
+        Color8A(0, 255*(by or 0.5)),
+        self.setPixelBlend)
 end
 
 function BB_mt.__index:copy()