Improve kmsdrm + rpi driver coexistence

* Ensure that DEFAULT_OGL is set correctly (fixes OpenGL)
* Silence kmsdrm driver init error when using VideoCore.
* Remove VideoCore cflags from SDL_CFLAGS; these flags are unnecessary
  and will cause conflicts when building SDL2 applications for VC4.

configure

@@ -19859,7 +19859,7 @@ $as_echo "$have_video_rpi" >&6; }
 
         if test x$have_video_rpi = xyes; then
             CFLAGS="$CFLAGS $RPI_CFLAGS"
-            SDL_CFLAGS="$SDL_CFLAGS $RPI_CFLAGS"
+            #SDL_CFLAGS="$SDL_CFLAGS $RPI_CFLAGS"
             EXTRA_CFLAGS="$EXTRA_CFLAGS $RPI_CFLAGS"
             EXTRA_LDFLAGS="$EXTRA_LDFLAGS $RPI_LIBS"
             SOURCES="$SOURCES $srcdir/src/video/raspberry/*.c"

configure.ac

@@ -1588,7 +1588,7 @@ AS_HELP_STRING([--enable-video-rpi], [use Raspberry Pi video driver [[default=ye
         
         if test x$have_video_rpi = xyes; then
             CFLAGS="$CFLAGS $RPI_CFLAGS"
-            SDL_CFLAGS="$SDL_CFLAGS $RPI_CFLAGS"
+            #SDL_CFLAGS="$SDL_CFLAGS $RPI_CFLAGS"
             EXTRA_CFLAGS="$EXTRA_CFLAGS $RPI_CFLAGS"
             EXTRA_LDFLAGS="$EXTRA_LDFLAGS $RPI_LIBS"
             SOURCES="$SOURCES $srcdir/src/video/raspberry/*.c"

src/video/SDL_egl.c

@@ -45,6 +45,8 @@
 
 #if SDL_VIDEO_DRIVER_RPI
 /* Raspbian places the OpenGL ES/EGL binaries in a non standard path */
+#include <unistd.h>
+#define DEFAULT_OGL ( vc4 ? "libGL.so.1" : "" )
 #define DEFAULT_EGL ( vc4 ? "libEGL.so.1" : "libbrcmEGL.so" )
 #define DEFAULT_OGL_ES2 ( vc4 ? "libGLESv2.so.2" : "libbrcmGLESv2.so" )
 #define ALT_EGL "libEGL.so"

src/video/kmsdrm/SDL_kmsdrmvideo.c

@@ -87,6 +87,12 @@ static int get_dricount(void)
 
     if (!(stat(KMSDRM_DRI_PATH, &sb) == 0
                 && S_ISDIR(sb.st_mode))) {
+#if SDL_VIDEO_DRIVER_RPI
+        // exit silently if VC4 driver is not active
+        SDL_bool vc4 = (0 == access("/sys/module/vc4/", F_OK));
+        if (!vc4)
+            return 0;
+#endif
         printf("The path %s cannot be opened or is not available\n",
                KMSDRM_DRI_PATH);
         return 0;