CI/CD should compile using latest clang with -Werror

Seems there are currently a few warnings that have snuck into master. Having a CI/CQ which runs with ToT/latest clang with -Werror to avoid stuff like that sneaking in would help integration efforts with other projects.

[21/90] Compiling C object 'tests/tests@@checkasm_bitdepth_8@sta/checkasm_looprestoration.c.o'.
../tests/checkasm/looprestoration.c: In function ‘check_wiener’:
../tests/checkasm/looprestoration.c:71:14: warning: passing argument 1 of ‘init_tmp’ from incompatible pointer type [-Wincompatible-pointer-types]
     init_tmp(left, 4 * sizeof(pixel), 4, 64);
              ^~~~
../tests/checkasm/looprestoration.c:35:13: note: expected ‘pixel * {aka unsigned char *}’ but argument is of type ‘pixel (*)[4] {aka unsigned char (*)[4]}’
 static void init_tmp(pixel *buf, const ptrdiff_t stride,
             ^~~~~~~~
[23/90] Compiling C object 'tests/tests@@checkasm_bitdepth_10@sta/checkasm_looprestoration.c.o'.
../tests/checkasm/looprestoration.c: In function ‘check_wiener’:
../tests/checkasm/looprestoration.c:71:14: warning: passing argument 1 of ‘init_tmp’ from incompatible pointer type [-Wincompatible-pointer-types]
     init_tmp(left, 4 * sizeof(pixel), 4, 64);
              ^~~~
../tests/checkasm/looprestoration.c:35:13: note: expected ‘pixel * {aka short unsigned int *}’ but argument is of type ‘pixel (*)[4] {aka short unsigned int (*)[4]}’
 static void init_tmp(pixel *buf, const ptrdiff_t stride,
             ^~~~~~~~
[31/90] Compiling C object 'src/src@@dav1d_bitdepth_8@sta/lf_apply.c.o'.
../src/lf_apply.c: In function ‘dav1d_loopfilter_sbrow_8bpc’:
../src/lf_apply.c:171:53: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for (unsigned y = starty4, mask = 1 << y; y < endy4; y++, mask <<= 1) {
                                                     ^
../src/lf_apply.c:179:66: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for (unsigned y = starty4 >> ss_ver, uv_mask = 1 << y; y < uv_endy4;
                                                                  ^
[33/90] Compiling C object 'src/src@@dav1d_bitdepth_10@sta/lf_apply.c.o'.
../src/lf_apply.c: In function ‘dav1d_loopfilter_sbrow_16bpc’:
../src/lf_apply.c:171:53: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for (unsigned y = starty4, mask = 1 << y; y < endy4; y++, mask <<= 1) {
                                                     ^
../src/lf_apply.c:179:66: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for (unsigned y = starty4 >> ss_ver, uv_mask = 1 << y; y < uv_endy4;
                                                                  ^
[76/90] Compiling C object 'src/src@@dav1d@sha/ref_mvs.c.o'.
../src/ref_mvs.c:351:33: warning: ‘default_warp_params’ defined but not used [-Wunused-const-variable=]
 static const WarpedMotionParams default_warp_params = {
                                 ^~~~~~~~~~~~~~~~~~~
../src/ref_mvs.c:321:22: warning: ‘num_8x8_blocks_high_lookup’ defined but not used [-Wunused-const-variable=]
 static const uint8_t num_8x8_blocks_high_lookup[BLOCK_SIZES_ALL] = {
                      ^~~~~~~~~~~~~~~~~~~~~~~~~~
../src/ref_mvs.c:309:22: warning: ‘num_8x8_blocks_wide_lookup’ defined but not used [-Wunused-const-variable=]
 static const uint8_t num_8x8_blocks_wide_lookup[BLOCK_SIZES_ALL] = {
                      ^~~~~~~~~~~~~~~~~~~~~~~~~~