Rename fill2d back to fill;

This rename was a mistake -- at the time I was thinking the fill shaders
were internal, but they definitely aren't.

src/api/l_graphics.c

@@ -56,7 +56,7 @@ StringEntry lovrDefaultShader[] = {
   [SHADER_FONT] = ENTRY("font"),
   [SHADER_CUBEMAP] = ENTRY("cubemap"),
   [SHADER_EQUIRECT] = ENTRY("equirect"),
-  [SHADER_FILL_2D] = ENTRY("fill2d"),
+  [SHADER_FILL_2D] = ENTRY("fill"),
   [SHADER_FILL_ARRAY] = ENTRY("fillarray"),
   [SHADER_LOGO] = ENTRY("logo"),
   { 0 }