Add hidpi test

Test hi-dpi "retina" display, where there are four times as many pixels as the window's (height x width) would have you believe.

On hi-res displays, the "HighRes" window will have one-pixel-wide vertical black bars within the red-to-purple gradient, while in the "LowRes" window they'll be two-pixel-wide. On regular displays, LowRes will have the one-pixel bars, and HiRes should be a down-sampled gradient with no bars.

CMakeLists.txt

@@ -276,6 +276,10 @@ if(MINIFB_BUILD_EXAMPLES)
             tests/multiple_windows.c
         )
 
+        add_executable(hidpi
+            tests/hidpi.c
+        )
+
     else()
 
         add_executable(noise

tests/hidpi.c

@@ -0,0 +1,48 @@
+#include <MiniFB.h>
+#include <stdio.h>
+#include <stdint.h>
+#include <unistd.h>
+#include <string.h>
+
+#define DIMEN_LOW      512
+static unsigned int g_buffer_low[DIMEN_LOW * DIMEN_LOW];
+
+#define DIMEN_HIGH      (2*DIMEN_LOW)
+static unsigned int g_buffer_high[DIMEN_HIGH * DIMEN_HIGH];
+
+void
+pretty_square(unsigned int *p, int dimen)
+{
+  memset(p, 127, dimen*dimen*4);
+  const int one_half_dimen = dimen / 2;
+  const int one_quarter_dimen = one_half_dimen / 2;
+  const int three_quarter_dimen = one_half_dimen + one_quarter_dimen;
+  for (int x = one_quarter_dimen; x < three_quarter_dimen; x++)
+    for (int y = one_quarter_dimen; y < three_quarter_dimen; y++)
+      p[y*dimen + x] = (x & 1) ? MFB_RGB(223,0,(255*(x-one_quarter_dimen))/one_half_dimen) : MFB_RGB(0,0,0);
+}
+
+int 
+main()
+{
+  pretty_square(g_buffer_low, DIMEN_LOW);
+  pretty_square(g_buffer_high, DIMEN_HIGH);
+
+  struct mfb_window *window_low = mfb_open("LowRes", DIMEN_LOW, DIMEN_LOW);
+  struct mfb_window *window_high = mfb_open("HighRes", DIMEN_HIGH/2, DIMEN_HIGH/2);
+
+  while (window_high || window_low)
+    {
+      if (window_low)
+	if (mfb_update_ex(window_low, g_buffer_low, DIMEN_LOW, DIMEN_LOW) != STATE_OK
+	      || !mfb_wait_sync(window_low))
+	    window_low = NULL;
+
+      if (window_high)
+	if (mfb_update_ex(window_high, g_buffer_high, DIMEN_HIGH, DIMEN_HIGH) != STATE_OK
+	    || !mfb_wait_sync(window_high))
+	  window_high = NULL;
+    }
+  
+  return 0;
+}