winsys: Expose environment variable to choose winsys

This makes it possible to override the winsys that cogl uses by setting
the COGL_RENDERER environment variable e.g. to "GLX" or "EGL"

cogl/cogl-renderer.c

@@ -28,6 +28,9 @@
 #include "config.h"
 #endif
 
+#include <stdlib.h>
+#include <string.h>
+
 #include "cogl.h"
 #include "cogl-internal.h"
 #include "cogl-object.h"
@@ -168,6 +171,7 @@ cogl_renderer_connect (CoglRenderer *renderer, GError **error)
 {
 #ifdef COGL_HAS_FULL_WINSYS
   int i;
+  char *renderer_name = getenv ("COGL_RENDERER");
 #endif
   GString *error_message;
 
@@ -180,6 +184,10 @@ cogl_renderer_connect (CoglRenderer *renderer, GError **error)
     {
       const CoglWinsysVtable *winsys = _cogl_winsys_vtable_getters[i]();
       GError *tmp_error = NULL;
+
+      if (renderer_name && strcmp (winsys->name, renderer_name) != 0)
+        continue;
+
       if (!winsys->renderer_connect (renderer, &tmp_error))
         {
           g_string_append_c (error_message, '\n');

cogl/winsys/cogl-winsys-egl.c

@@ -1082,6 +1082,7 @@ _cogl_winsys_context_egl_get_egl_display (CoglContext *context)
 
 static CoglWinsysVtable _cogl_winsys_vtable =
   {
+    .name = "EGL",
     .get_proc_address = _cogl_winsys_get_proc_address,
     .renderer_connect = _cogl_winsys_renderer_connect,
     .renderer_disconnect = _cogl_winsys_renderer_disconnect,

cogl/winsys/cogl-winsys-glx.c

@@ -1880,6 +1880,7 @@ _cogl_winsys_texture_pixmap_x11_get_texture (CoglTexturePixmapX11 *tex_pixmap)
 
 static CoglWinsysVtable _cogl_winsys_vtable =
   {
+    .name = "GLX",
     .get_proc_address = _cogl_winsys_get_proc_address,
     .renderer_connect = _cogl_winsys_renderer_connect,
     .renderer_disconnect = _cogl_winsys_renderer_disconnect,

cogl/winsys/cogl-winsys-private.h

@@ -54,6 +54,8 @@ typedef enum
 
 typedef struct _CoglWinsysVtable
 {
+  const char *name;
+
   CoglFuncPtr
   (*get_proc_address) (const char *name);