Fixed negative/flipped axis matrix drawing.

src/pixie/images.nim

@@ -881,7 +881,10 @@ proc drawUber(
                 x += 16
 
       var srcPos = p + dx * x.float32 + dy * y.float32
-      srcPos = vec2(max(0, srcPos.x), max(0, srcPos.y))
+      srcPos = vec2(
+        clamp(srcPos.x, 0, b.width.float32),
+        clamp(srcPos.y, 0, b.height.float32)
+      )
 
       for x in x ..< xMax:
         let samplePos = ivec2((srcPos.x - h).int32, (srcPos.y - h).int32)

tests/test_images.nim

@@ -202,3 +202,16 @@ block:
   let image = newImage(100, 100)
   image.fill(rgba(255, 255, 255, 255))
   doAssert not image.isTransparent()
+
+block:
+  let a = newImage(400, 400)
+  let b = newImage(156, 434)
+  b.fill(rgba(255, 0, 0, 255))
+  a.draw(
+    b,
+    mat3(
+      -0.5, -4.371138828673793e-008, 0.0,
+      -4.371138828673793e-008, 0.5, 0.0,
+      292.0, 45.0, 1.0
+    )
+  )