osx - require full opengl for gl support on osx

the default still is gl-es as documented. you need to explicitly turn
on full opengl to get gl support on osx because thats all the engine
does. document this exception for osx (having gl magically being full
on osx and gle-es elsewhere is another option but then we need
different os specific defaults for this which is kind of bad in other
ways).

README

@@ -464,7 +464,7 @@ Required by default:
   * libxcursor
   * libxp
   * libxi (2.2 or newer)
-  * opengl(mesa etc.) (opengl/glx/full or opengl-es2/egl)
+  * opengl(mesa etc.) (opengl/glx/full or opengl-es2/egl. full opengl only on osx - must be explicitly specified to be full to have support)
   * giflib
   * util-linux (limbount + libblkid)
   * systemd / libudev

src/modules/evas/engines/meson.build

@@ -13,19 +13,29 @@ if sys_windows == true
  ]
 endif
 
+have_gl_engine = false
+
 if get_option('opengl') != 'none'
   engines += [
-   ['gl_generic', []],
    ['gl_x11', ['x11']],
-   ['gl_drm', ['drm']],
-   ['gl_cocoa', ['cocoa']],
+   ['gl_drm', ['drm']]
   ]
+  have_gl_engine = true
+endif
+
+if get_option('opengl') == 'full'
+  engines += [['gl_cocoa', ['cocoa']]]
+  have_gl_engine = true
 endif
 
 if get_option('opengl') == 'es-egl'
   engines += [['wayland_egl', ['wl']]]
+  have_gl_engine = true
 endif
 
+if have_gl_engine
+  engines += [['gl_generic', []]]
+endif
 
 foreach engine_conf : engines
   engine = engine_conf[0]