q3map2: add -externalnames option to write explicit external lightmap names when...

q3map2: add -externalnames option to write explicit external lightmap names when writing style shaders

It makes possible for game engines to run less code on each rendering
frame by not doing any computation to find the file.

On some game engines it may even make the lightmaps be processed
with simpler GLSL code as they will be processed as simple colormaps
instead of more complex code purposed for usual lightmaps with
features that aren't needed in such situation anyway.

Or some engine may implement a specific GLSL code to blend multiple
colormaps in a single pass without having to implement alternate code
for lightmaps.

Example with scripts/q3map2_gloom2.shader:

```
-gloom2/7C9AD47A87BC98CCA1FF30C4D788DD47
+gloom2/72353935F9542782B847B2E329A8CC80
 { // Q3Map2 defaulted
 	{
 		map $lightmap
 		rgbGen identity
 	}

 	// Q3Map2 custom lightstyle stage(s)
 	{
-		map $lightmap
+		map maps/gloom2/lm_0000.tga
 		blendFunc GL_SRC_ALPHA GL_ONE
 		rgbGen wave noise 1 .75 1.6 4.2 // style 2
 		tcGen lightmap
 		tcMod transform 1 0 0 1 0.54492 0.10156
 	}
 	{
-		map $lightmap
+		map maps/gloom2/lm_0000.tga
 		blendFunc GL_SRC_ALPHA GL_ONE
 		rgbGen wave noise 1 .5 3.7 4.9 // style 3
 		tcGen lightmap
 		tcMod transform 1 0 0 1 -0.38672 0.19043
 	}
 	{
-		map $lightmap
+		map maps/gloom2/lm_0000.tga
 		blendFunc GL_SRC_ALPHA GL_ONE
 		rgbGen wave noise 1 1 2.6 1.3 // style 4
 		tcGen lightmap
 		tcMod transform 1 0 0 1 0.24023 0.19043
 	}

 	{
 		map textures/gloom2/e8clangfloor05c.tga
 		blendFunc GL_DST_COLOR GL_ZERO
 		rgbGen identity
 	}
 }
```