Delete whole utf8 character on backspace

[johanmalm]

...rather than just the last byte of the password buffer.

Demostrate the need for this by taking the following steps:

• Apply the patch below (before this commit).
• Set keyboard layout to one where utf8 characters longer than one byte can be obtained, for example åäö using Swedish layout (XKB_DEFAULT_LAYOUT=se).
• Type "åäö" then press backspace and observe that it takes two backspaces to delete each character fully, and therefore six backspaces to fully clear the password buffer.

diff --git a/password.c b/password.c
index e1a1d9a..b640cd3 100644
--- a/password.c
+++ b/password.c
@@ -29,6 +29,7 @@ void clear_password_buffer(struct swaylock_password *pw) {
 static bool backspace(struct swaylock_password *pw) {
    if (pw->len != 0) {
        pw->buffer[--pw->len] = 0;
+       fprintf(stderr, "%s\n", pw->buffer);
        return true;
    }
    return false;