Complement fixup for 4a1feaa555 (texture shaders)

We now multiply the alpha by the image alpha (as per severin
suggestion).
That still doesn't solve the main question of user preference drawing
black.

source/blender/gpu/shaders/gpu_shader_2D_texture_2D_frag.glsl

@@ -11,5 +11,6 @@ uniform sampler2D texture_map;
 
 void main()
 {
-	fragColor = vec4(texture2D(texture_map, texture_coord).rgb, alpha);
+	fragColor = texture2D(texture_map, texture_coord)
+	fragColor.a *= alpha;
 }

source/blender/gpu/shaders/gpu_shader_2D_texture_rect_frag.glsl

@@ -11,5 +11,6 @@ uniform sampler2DRect texture_map;
 
 void main()
 {
-	fragColor = vec4(texture2DRect(texture_map, texture_coord).rgb, alpha);
+	fragColor = texture2DRect(texture_map, texture_coord);
+	fragColor.a *= alpha;
 }