Fix T76909 GPencil: Layers with mask(s) invisible when using "in front"

The gpSceneDepthTexture is using a dummy 1px texture which was generating
wrong values for uvs when sampling gpMaskTexture.

Use the max size of both since any of the 2 can use dummy texture.

source/blender/draw/engines/gpencil/shaders/gpencil_frag.glsl

@@ -99,8 +99,10 @@ void main()
     discard;
   }
 
+  vec2 fb_size = max(vec2(textureSize(gpSceneDepthTexture, 0).xy),
+                     vec2(textureSize(gpMaskTexture, 0).xy));
+  vec2 uvs = gl_FragCoord.xy / fb_size;
   /* Manual depth test */
-  vec2 uvs = gl_FragCoord.xy / vec2(textureSize(gpSceneDepthTexture, 0).xy);
   float scene_depth = texture(gpSceneDepthTexture, uvs).r;
   if (gl_FragCoord.z > scene_depth) {
     discard;