Workaround for get_proc_address failing

Retroarch currently has an issue where get_proc_address call fails on
the Raspberry Pi (https://github.com/libretro/RetroArch/issues/6137). If
the call fails, we fall back to loading symbols from libGLESv2.so, if
available.

Tested working on the Pi 3 with buildroot.

Signed-off-by: Mahyar Koshkouei <mahyar.koshkouei@gmail.com>

Makefile

@@ -118,7 +118,7 @@ else
 endif
 
 OBJECTS	:= mpv-libretro.o
-LDFLAGS	+= -lmpv
+LDFLAGS	+= -lmpv -ldl
 CFLAGS	+= -Wall -pedantic
 
 all: $(TARGET)

mpv-libretro.c

@@ -15,6 +15,7 @@
  * along with this program. If not, see <http://www.gnu.org/licenses/>.
  */
 
+#include <dlfcn.h>
 #include <stdarg.h>
 #include <stdio.h>
 #include <stdlib.h>
@@ -122,6 +123,20 @@ static void *get_proc_address_mpv(void *fn_ctx, const char *name)
 	void *proc_addr = (void *) hw_render.get_proc_address(name);
 #pragma GCC diagnostic pop
 
+    // EGL 1.4 (supported by the RPI firmware) does not necessarily return
+    // function pointers for core functions.
+    if (!proc_addr) {
+		void *h = dlopen("/opt/vc/lib/libGLESv2.so", RTLD_LAZY);
+
+		if (!h)
+			h = dlopen("/usr/lib/libGLESv2.so", RTLD_LAZY);
+
+        if (h) {
+            proc_addr = dlsym(h, name);
+            dlclose(h);
+        }
+    }
+
 	if(proc_addr == NULL)
 		log_cb(RETRO_LOG_ERROR, "Failure obtaining %s proc address\n", name);
 
@@ -287,6 +302,7 @@ static void context_reset(void)
 		goto err;
 	}
 
+
 	/* Keep trying until mpv accepts the property. This is done to seek to the
 	 * point in the file after the previous context was destroyed. If no
 	 * context was destroyed previously, the file seeks to 0.