Fix inconsistent handling of transparent colors in the middle of palettes

test/gfx/alpha_embedded.err

@@ -1,4 +0,0 @@
-error: Failed to fit tile colors [GB:07,09,05; GB:12,14,05; GB:20,21,06] in specified palettes
-note: The following palette was specified:
-      - [GB:20,21,06; GB:12,14,05]
-Conversion aborted after 1 error

test/gfx/middle_transparent_embedded.err

@@ -0,0 +1,2 @@
+error: Each palette spec can only contain up to 3 colors plus the implict transparent color
+Conversion aborted after 1 error

test/gfx/middle_transparent_embedded.flags

@@ -0,0 +1 @@
+-c embedded

test/gfx/middle_transparent_gbc_spec.err

@@ -0,0 +1,2 @@
+error: Each palette spec can only contain up to 3 colors plus the implict transparent color
+Conversion aborted after 1 error

test/gfx/middle_transparent_gbc_spec.flags

@@ -0,0 +1 @@
+-c gbc:middle_transparent_gbc_spec.pal

test/gfx/middle_transparent_png_spec.err

@@ -0,0 +1,2 @@
+error: Each palette spec can only contain up to 3 colors plus the implict transparent color
+Conversion aborted after 1 error

test/gfx/middle_transparent_png_spec.flags

@@ -0,0 +1 @@
+-c png:middle_transparent_png_spec.pal.png