Kernel: Respect actual framebuffer pitch

Instead of winging it with "width * 4", use the actual pitch since it
may be different.

This makes the kernel text console show up in native 1368x768 on my
ThinkPad X250. :^)

Kernel/Graphics/Console/GenericFramebufferConsole.h

@@ -45,6 +45,7 @@ protected:
     {
     }
     virtual u8* framebuffer_data() = 0;
+    size_t framebuffer_pitch() const { return m_pitch; }
     virtual void clear_glyph(size_t x, size_t y);
     size_t m_pitch;
 };