evas/engines/gl_generic: remove multiclass inheritance

evas_ector_gl_buffer/evas_ector_gl_image_buffer contained a regular
class in extensions list, which is wrong.

src/lib/ector/gl/ector_gl_buffer.eo

@@ -1,4 +1,4 @@
-class Ector.GL.Buffer (Efl.Object, Ector.Buffer)
+class Ector.GL.Buffer extends Efl.Object implements Ector.Buffer
 {
    [[Ector GL buffer class]]
    data: null;

src/modules/evas/engines/gl_generic/evas_ector_gl_buffer.eo

@@ -1,4 +1,4 @@
-class Evas.Ector.GL.Buffer (Efl.Object, Evas.Ector.Buffer, Ector.GL.Buffer)
+class Evas.Ector.GL.Buffer extends Ector.GL.Buffer implements Evas.Ector.Buffer
 {
    [[An Ector GL buffer capable of being mapped, drawn and rendered to.
 

src/modules/evas/engines/gl_generic/evas_ector_gl_image_buffer.eo

@@ -1,4 +1,4 @@
-class Evas.Ector.GL.Image.Buffer (Efl.Object, Evas.Ector.Buffer, Ector.GL.Buffer)
+class Evas.Ector.GL.Image.Buffer extends Ector.GL.Buffer implements Evas.Ector.Buffer
 {
    [[An Ector GL buffer wrapping an existing Evas_GL_Image.]]
    implements {