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

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.

The option is believed to be only safe when external lightmaps are used, so using -externalnames implicitly enables -external.

Example with scripts/q3map2_gloom2.shader produced when compiling the Gloom2 map for Unvanquished, the diff is before/after this patch:

-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
 	}
 }

-gloom2/15F4DE8571DF6BA4B8F54BE06445A8A3
+gloom2/EC8E06CEB8488CD5FCDFCC6591453C89
 { // Q3Map2 defaulted
 	{
 		map $lightmap
 		rgbGen identity
 	}
 	
 	// Q3Map2 custom lightstyle stage(s)
 	{
-		map $lightmap
+		map maps/gloom2/lm_0002.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 -0.50000 0.50000
 	}
 	{
-		map $lightmap
+		map maps/gloom2/lm_0002.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 -0.21777 0.02637
 	}
 
 	{
 		map textures/gloom2/floor1_l3_1a.tga
 		blendFunc GL_DST_COLOR GL_ZERO
 		rgbGen identity
 	}
 }

Note that q3map2 already writes the full path in some situations, here this shader is already written this way before this patch, so we know engine supporting light styles are already expected to support this way of doing lightstyles:

gloom2/4AE931928BA2A9E2B778C73587A06049
{ // Q3Map2 defaulted
	{
		map $lightmap
		rgbGen identity
	}
	
	// Q3Map2 custom lightstyle stage(s)
	{
		map maps/gloom2/lm_0002.tga
		blendFunc GL_SRC_ALPHA GL_ONE
		rgbGen wave noise 1 .5 3.7 4.9 // style 3
		tcGen lightmap
	}

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