derive default line width from display dpi

allows better default on high dpi displays where the previous
default of 1 pixel would've been too small.

Closes: #285

configure.ac

@@ -28,7 +28,7 @@ AS_IF([test "x$orig_CFLAGS" = "x"], [
         AX_APPEND_LINK_FLAGS(["-flto"])
     ])
     m4_foreach([SCROT_FLAG],
-        [["-O3"], ["-Wall"], ["-Wextra"], ["-Wpedantic"]], [
+        [["-O3"], ["-Wall"], ["-Wextra"], ["-Wpedantic"], ["-fno-math-errno"]], [
             AX_APPEND_COMPILE_FLAGS(["SCROT_FLAG"])
             AS_IF([test "x$LTO_ENABLED" = "xyes"], [
                 AX_APPEND_LINK_FLAGS(["SCROT_FLAG"])

deps.pc

@@ -2,4 +2,5 @@ Name: scrot's mandatory dependencies
 Description: ditto
 Version: infinite
 Cflags: -D_XOPEN_SOURCE=700L
+Libs: -lm
 Requires: x11 imlib2 >= 1.11.0 xcomposite >= 0.2.0 xext xfixes >= 5.0.1 xinerama >= 1.1.3

man/scrot.txt

@@ -175,7 +175,7 @@ SELECTION STYLE
 
   Without the -l option, a default style is used:
 
-    mode=auto,style=solid,width=1,opacity=100
+    mode=auto,style=solid,width=$dpi/75,opacity=100
 
   Example:
 

src/options.c

@@ -68,7 +68,6 @@ struct ScrotOptions opt = {
     .quality = 75,
     .compression = 7,
     .lineStyle = LineSolid,
-    .lineWidth = 1,
     .lineOpacity = SELECTION_OPACITY_DEFAULT,
     .stackDirection = HORIZONTAL,
     .outputFile = defaultOutputFile,

src/scrot_selection.c

@@ -35,6 +35,7 @@ CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 
 #include <err.h>
 #include <errno.h>
+#include <math.h>
 #include <stdint.h>
 #include <stdlib.h>
 #include <string.h>
@@ -423,6 +424,14 @@ Imlib_Image scrotSelectionSelectMode(void)
             opt.lineMode = LINE_MODE_CLASSIC;
     }
 
+    if (opt.lineWidth == 0) {
+        double mmToInches = 1 / 25.4;
+        double inchesDiag = hypot(scr->mwidth, scr->mheight) * mmToInches;
+        double pixelsDiag = hypot(scr->width, scr->height);
+        double dpi = round(pixelsDiag / inchesDiag);
+        opt.lineWidth = fmin(fmax(round(dpi / 75.0), 1), 8);
+    }
+
     if (opt.delaySelection)
         scrotDoDelay();