Fix T74229: EEVEE Bloom + Transparency

Bug was introduced by the render passes. We had to tweak the bloom
shader a bit so we could reuse it. After that tweaking the original
alpha was ignored.

This patch will read and store the correct alpha channel.

source/blender/draw/engines/eevee/shaders/effect_bloom_frag.glsl

@@ -202,9 +202,9 @@ vec4 step_resolve(void)
 #else
   vec3 blur = upsample_filter(sourceBuffer, uvcoordsvar.xy, sourceBufferTexelSize);
 #endif
-  vec3 base = bloomAddBase ? textureLod(baseBuffer, uvcoordsvar.xy, 0.0).rgb : vec3(0.0);
-  vec3 cout = base + blur * bloomColor;
-  return vec4(cout, 1.0);
+  vec4 base = bloomAddBase ? textureLod(baseBuffer, uvcoordsvar.xy, 0.0) : vec4(0.0);
+  vec3 cout = base.rgb + blur * bloomColor;
+  return vec4(cout, base.a);
 }
 
 void main(void)