evas gl: Properly verify that Evas GL works

For SW engine we need to verify that OSMesa is present. The patch
fb048e7312 broke the logic.

Tested by temporarily removing OSMesa from my system.

Fixes T6617 (again)

src/lib/evas/canvas/evas_gl.c

@@ -147,7 +147,10 @@ evas_gl_new(Evas *e)
    evas_gl->evas = efl_data_ref(e, EVAS_CANVAS_CLASS);
    LKI(evas_gl->lck);
 
-   if (!evas_gl->evas->engine.func->gl_context_create)
+   if (!evas_gl->evas->engine.func->gl_context_create ||
+       !evas_gl->evas->engine.func->gl_supports_evas_gl ||
+       !evas_gl->evas->engine.func->gl_supports_evas_gl(
+          _evas_engine_context(evas_gl->evas)))
      {
         ERR("Evas GL engine not available.");
         efl_data_unref(e, evas_gl->evas);

src/lib/evas/include/evas_private.h

@@ -1432,6 +1432,7 @@ struct _Evas_Func
    int  (*font_right_inset_get)            (void *engine, Evas_Font_Set *font, const Evas_Text_Props *text_props);
 
    /* EFL-GL Glue Layer */
+   Eina_Bool (*gl_supports_evas_gl)      (void *engine);
    void *(*gl_output_set)                (void *engine, void *output);
    void *(*gl_surface_create)            (void *engine, void *config, int w, int h);
    void *(*gl_pbuffer_surface_create)    (void *engine, void *config, int w, int h, int const *attrib_list);

src/modules/evas/engines/gl_generic/evas_engine.c

@@ -1528,6 +1528,14 @@ evgl_init(Render_Engine_GL_Generic *engine)
 
 #define EVGLINIT(_ret) Render_Output_GL_Generic *re; if ((re = evgl_init(engine)) == NULL) return _ret
 
+static Eina_Bool
+eng_gl_supports_evas_gl(void *engine EINA_UNUSED)
+{
+   // Evas GL should always work... But let's do a full init anyway.
+   EVGLINIT(EINA_FALSE);
+   return EINA_TRUE;
+}
+
 static void *
 eng_gl_output_set(void *eng, void *output)
 {
@@ -3166,6 +3174,7 @@ module_open(Evas_Module *em)
    ORD(font_cache_set);
    ORD(font_cache_get);
 
+   ORD(gl_supports_evas_gl);
    ORD(gl_output_set);
    ORD(gl_surface_create);
    ORD(gl_pbuffer_surface_create);

src/modules/evas/engines/software_generic/evas_engine.c

@@ -3461,13 +3461,19 @@ _tls_check(void)
 }
 #endif
 
-static inline int
+static inline Eina_Bool
 _check_gl(void)
 {
    if (!gl_lib_init()) return 0;
    return 1;
 }
 
+static Eina_Bool
+eng_gl_supports_evas_gl(void *data EINA_UNUSED)
+{
+   return _check_gl();
+}
+
 static void *
 eng_gl_surface_create(void *data EINA_UNUSED, void *config, int w, int h)
 {
@@ -4815,6 +4821,7 @@ static Evas_Func func =
      eng_font_pen_coords_get,
      eng_font_text_props_info_create,
      eng_font_right_inset_get,
+     eng_gl_supports_evas_gl, // returns true iif OSMesa is present
      NULL, // No need to set output for software engine
      eng_gl_surface_create, // need software mesa for gl rendering <- gl_surface_create
      NULL, // need software mesa for gl rendering <- gl_pbuffer_surface_create