Initial shot at a renderer target for Apple's Metal API.

This isn't complete, but is enough to run testsprite2. It's currently
Mac-only; with a little work to figure out how to properly glue in a Metal
layer to a UIView, this will likely work on iOS, too.

This is only wired up to the configure script right now, and disabled by
default. CMake and Xcode still need their bits filled in as appropriate.

configure.in

@@ -2053,6 +2053,19 @@ AC_HELP_STRING([--enable-video-cocoa], [use Cocoa video driver [[default=yes]]])
     fi
 }
 
+CheckMETAL()
+{
+    AC_ARG_ENABLE(render-metal,
+AC_HELP_STRING([--enable-render-metal], [enable the Metal render driver [[default=no]]]),
+                                , enable_render_metal=no)
+    if test x$enable_render = xyes -a x$enable_render_metal = xyes; then
+        dnl This should maybe make sure you have a supported SDK version.
+        AC_DEFINE(SDL_VIDEO_RENDER_METAL, 1, [ ])
+        SOURCES="$SOURCES $srcdir/src/render/metal/*.m"
+    fi
+}
+
+
 dnl Find DirectFB
 CheckDirectFB()
 {
@@ -2103,7 +2116,6 @@ AC_HELP_STRING([--enable-directfb-shared], [dynamically load directfb support [[
                               , enable_directfb_shared=yes)
 
             AC_DEFINE(SDL_VIDEO_DRIVER_DIRECTFB, 1, [ ])
-            AC_DEFINE(SDL_VIDEO_RENDER_DIRECTFB, 1, [ ])
             SOURCES="$SOURCES $srcdir/src/video/directfb/*.c"
             EXTRA_CFLAGS="$EXTRA_CFLAGS $DIRECTFB_CFLAGS"
 
@@ -3636,8 +3648,9 @@ AC_HELP_STRING([--enable-render-d3d], [enable the Direct3D render driver [[defau
         CheckDiskAudio
         CheckDummyAudio
         CheckDLOPEN
-        CheckPTHREAD
+        CheckMETAL
         CheckVulkan
+        CheckPTHREAD
 
         # Set up files for the audio library
         if test x$enable_audio = xyes; then
@@ -3701,6 +3714,10 @@ AC_HELP_STRING([--enable-render-d3d], [enable the Direct3D render driver [[defau
         EXTRA_LDFLAGS="$EXTRA_LDFLAGS -Wl,-framework,OpenGLES"
         EXTRA_LDFLAGS="$EXTRA_LDFLAGS -Wl,-framework,QuartzCore"
         EXTRA_LDFLAGS="$EXTRA_LDFLAGS -Wl,-framework,UIKit"
+
+        if test x$enable_render = xyes -a x$enable_render_metal = xyes; then
+            EXTRA_LDFLAGS="$EXTRA_LDFLAGS -Wl,-framework,Metal"
+        fi
         ;;
     *-*-darwin* )
         # This could be either full "Mac OS X", or plain "Darwin" which is
@@ -3719,6 +3736,7 @@ AC_HELP_STRING([--enable-render-d3d], [enable the Direct3D render driver [[defau
         CheckDummyAudio
         CheckDLOPEN
         CheckCOCOA
+        CheckMETAL
         CheckX11
         CheckMacGL
         CheckOpenGLX11