LibGfx: Fix off-by-some in Painter::draw_scaled_bitmap_with_transform()

Before this, drawing a 1x1 bitmap scaled up to MxN would only fill
M/2 x N/2 pixel, due to source_point going outside (0, 0).

Tests/LibGfx/CMakeLists.txt

@@ -6,6 +6,7 @@ set(TEST_SOURCES
     TestGfxBitmap.cpp
     TestICCProfile.cpp
     TestImageDecoder.cpp
+    TestPainter.cpp
     TestParseISOBMFF.cpp
     TestRect.cpp
     TestScalingFunctions.cpp