Add support for new blur method dual_kawase [WIP]

**Work-in-Progress**

Add `dual_kawase` to configuration and argument parsing. Allow `kawase`
for backward compatibility. Add `--blur-strength` parameter for
blur-method `dual_kawase`.

Update documentation to reflect the new blur-method and parameters.

src/backend/backend.h

@@ -5,8 +5,8 @@
 
 #include <stdbool.h>
 
-#include "config.h"
 #include "compiler.h"
+#include "config.h"
 #include "driver.h"
 #include "kernel.h"
 #include "region.h"
@@ -65,6 +65,11 @@ struct kernel_blur_args {
 	int kernel_count;
 };
 
+struct dual_kawase_blur_args {
+	int size;
+	int strength;
+};
+
 struct backend_operations {
 	// ===========    Initialization    ===========
 

src/backend/gl/gl_common.c

@@ -977,6 +977,12 @@ void *gl_create_blur_context(backend_t *base, enum blur_method method, void *arg
 		return ctx;
 	}
 
+	if (method == BLUR_METHOD_DUAL_KAWASE) {
+		log_warn("Blur method 'dual_kawase' is not yet implemented.");
+		ctx->method = BLUR_METHOD_NONE;
+		return ctx;
+	}
+
 	int nkernels;
 	ctx->method = BLUR_METHOD_KERNEL;
 	if (method == BLUR_METHOD_KERNEL) {

src/backend/xrender/xrender.c

@@ -507,6 +507,12 @@ void *create_blur_context(backend_t *base attr_unused, enum blur_method method,
 		ret->method = BLUR_METHOD_NONE;
 		return ret;
 	}
+	if (method == BLUR_METHOD_DUAL_KAWASE) {
+		log_warn("Blur method 'dual_kawase' is not compatible with the 'xrender' "
+		         "backend.");
+		ret->method = BLUR_METHOD_NONE;
+		return ret;
+	}
 
 	ret->method = BLUR_METHOD_KERNEL;
 	struct conv **kernels;