renderer,config: add custom DRM modeline support (#2254)

This allows specifying custom display resolutions for the DRM backend.
This is useful for display overclocking, working around broken EDIDs,
etc. To use this feature, specify a modeline instead of a resolution
in the config, for example:

    monitor = DP-1, modeline 1071.101 3840 3848 3880 3920 2160 2263 2271 2277 +hsync -vsync, 0x0, 1

This example is a custom 3840x2160@120Hz mode with tightened timings.
I use it because the standard timings don't work with my monitor and GPU
combination (M28U with RX580).

The syntax is compatible with Sway and Xorg.

src/config/ConfigManager.hpp

@@ -11,6 +11,7 @@
 #include <algorithm>
 #include <regex>
 #include <optional>
+#include <xf86drmMode.h>
 #include "../Window.hpp"
 #include "../helpers/WLClasses.hpp"
 
@@ -44,6 +45,7 @@ struct SMonitorRule {
     wl_output_transform transform   = WL_OUTPUT_TRANSFORM_NORMAL;
     std::string         mirrorOf    = "";
     bool                enable10bit = false;
+    drmModeModeInfo     drmMode     = {};
 };
 
 struct SWorkspaceRule {