Implement ETC1S/ETC2AS image compression

Explanation:

ETC1S encoding is a subset of ETC1, which is using only one color endpoint per 4x4 block (modifier indices are identical for both subblocks, base color is encoded differentially as RGB555 with the differential RGB333 part always set to zero, flip bit is always set to zero).
Usage: crunch_x64.exe -ETC1S input.png -out output.ktx

ETC2AS encoding is a subset of ETC2A encoding which is using ETC1S encoding for color and ETC2A encoding for alpha.
Usage: crunch_x64.exe -ETC2AS input.png -out output.ktx

crnlib/crn_etc.cpp

@@ -1580,4 +1580,47 @@ uint64 pack_etc1_block(etc1_block& dst_block, const color_quad_u8* pSrc_pixels,
   return best_error;
 }
 
+uint64 pack_etc1s_block(etc1_block& dst_block, const color_quad_u8* pSrc_pixels, crn_etc1_pack_params& pack_params) {
+  uint8 selectors[16];
+  etc1_optimizer optimizer;
+  etc1_optimizer::params params;
+  params.m_pSrc_pixels = pSrc_pixels;
+  params.m_num_src_pixels = 16;
+  params.m_use_color4 = false;
+  params.m_constrain_against_base_color5 = false;
+  etc1_optimizer::results results;
+  results.m_pSelectors = selectors;
+  results.m_n = 16;
+  optimizer.init(params, results);
+
+  const int scan[] = {-4, -3, -2, -1, 0, 1, 2, 3, 4};
+  params.m_scan_delta_size = pack_params.m_quality == cCRNETCQualitySlow ? CRNLIB_ARRAY_SIZE(scan) : pack_params.m_quality == cCRNETCQualityMedium ? 3 : 1;
+  params.m_pScan_deltas = scan + ((CRNLIB_ARRAY_SIZE(scan) - params.m_scan_delta_size) >> 1);
+  optimizer.compute();
+
+  if (params.m_quality >= cCRNETCQualityMedium && results.m_error > 6000) {
+    const int refine_medium[] = {-3, -2, 2, 3};
+    const int refine_high[] = {-8, -7, -6, -5, 5, 6, 7, 8};
+    if (params.m_quality == cCRNETCQualityMedium) {
+      params.m_scan_delta_size = CRNLIB_ARRAY_SIZE(refine_medium);
+      params.m_pScan_deltas = refine_medium;
+    } else {
+      params.m_scan_delta_size = results.m_error > 12000 ? CRNLIB_ARRAY_SIZE(refine_high) : 2;
+      params.m_pScan_deltas = refine_high + ((CRNLIB_ARRAY_SIZE(refine_high) - params.m_scan_delta_size) >> 1);
+    }
+    optimizer.compute();
+  }
+
+  uint32 selector = 0;
+  for (uint32 i = 0, t = 8, h = 0; h < 4; h++, t -= 15) {
+    for (uint32 w = 0; w < 4; w++, t += 4, i++) {
+      uint32 s = g_selector_index_to_etc1[selectors[i]];
+      selector |= (s >> 1 | (s & 1) << 16) << (t & 15);
+    }
+  }
+
+  dst_block.m_uint64 = (uint64)selector << 32 | results.m_block_inten_table << 29 | results.m_block_inten_table << 26 | 1 << 25 | (results.m_block_color_unscaled.m_u32 & 0xFFFFFF) << 3;
+  return results.m_error;
+}
+
 }  // namespace crnlib