eet: set alpha_texture for ETC1+Alpha

Summary:
alpha_texture need to be set EINA_TRUE to support ETC1+Alpha

@fix

Test Plan:
Create an EDC file with png image with Alpha.
           compress image with ETC1 and ETC2.
           Observe Alpha is properly applied in both case.

Reviewers: jpeg, Hermet, cedric

Reviewed By: cedric

Subscribers: conr2d

Differential Revision: https://phab.enlightenment.org/D4307

Signed-off-by: Cedric BAIL <cedric@osg.samsung.com>

src/lib/eet/eet_image.c

@@ -855,6 +855,7 @@ eet_data_image_etc1_compressed_convert(int         *size,
         etc_block_size = 8;
         num_planes = 2; // RGB and Alpha
         header[5] = 3;
+        alpha_texture = EINA_TRUE;
         codec = "ETC1+Alpha";
         break;
       default: abort();