LibGfx/JBIG2: Simplify and restrict adaptive template pixel reading

EXTTEMPLATE=1 was added later and doesn't seem to be used much in
practice -- it doesn't appear in no simple generic regions in any PDF
I tested so far at least. Since the spec contradicts itself on what
to do with these as far as I can tell, error out on them for now and
then add support once we find actual files using this, so that we can
check our implementation actually works.

Deduplicate the data reading for the different cases, and
zero-initialize all adaptive template pixels to zero to make that
possible.

Other than prohibiting EXTTEMPLATE=1, no behavior change.

Userland/Libraries/LibGfx/ImageFormats/JBIG2Loader.cpp

@@ -597,40 +597,25 @@ static ErrorOr<void> decode_immediate_generic_region(JBIG2LoadingContext& contex
     data = data.slice(sizeof(flags));
 
     // 7.4.6.3 Generic region segment AT flags
-    GenericRegionDecodingInputParameters::AdaptiveTemplatePixel adaptive_template_pixels[12];
+    GenericRegionDecodingInputParameters::AdaptiveTemplatePixel adaptive_template_pixels[12] = {};
     if (!uses_mmr) {
         dbgln_if(JBIG2_DEBUG, "Non-MMR generic region, GBTEMPLATE={} TPGDON={} EXTTEMPLATE={}", arithmetic_coding_template, typical_prediction_generic_decoding_on, uses_extended_reference_template);
 
-        if (arithmetic_coding_template == 0 && !uses_extended_reference_template) {
-            if (data.size() < 8)
-                return Error::from_string_literal("JBIG2ImageDecoderPlugin: No adaptive template data");
-            for (int i = 0; i < 4; ++i) {
-                adaptive_template_pixels[i].x = static_cast<i8>(data[2 * i]);
-                adaptive_template_pixels[i].y = static_cast<i8>(data[2 * i + 1]);
-            }
-            data = data.slice(8);
-        } else if (arithmetic_coding_template == 0 && uses_extended_reference_template) {
-            if (data.size() < 24)
-                return Error::from_string_literal("JBIG2ImageDecoderPlugin: No adaptive template data");
-            for (int i = 0; i < 12; ++i) {
-                adaptive_template_pixels[i].x = static_cast<i8>(data[2 * i]);
-                adaptive_template_pixels[i].y = static_cast<i8>(data[2 * i + 1]);
-            }
-            // FIXME: Is this supposed to be 24 or 32? THe spec says "32-byte field as shown below" and then shows 24 bytes.
-            data = data.slice(24);
-        } else {
-            if (data.size() < 2)
-                return Error::from_string_literal("JBIG2ImageDecoderPlugin: No adaptive template data");
-            for (int i = 0; i < 1; ++i) {
-                adaptive_template_pixels[i].x = static_cast<i8>(data[2 * i]);
-                adaptive_template_pixels[i].y = static_cast<i8>(data[2 * i + 1]);
-            }
-            for (int i = 1; i < 4; ++i) {
-                adaptive_template_pixels[i].x = 0;
-                adaptive_template_pixels[i].y = 0;
-            }
-            data = data.slice(2);
+        if (arithmetic_coding_template == 0 && uses_extended_reference_template) {
+            // This was added in T.88 Amendment 2 (https://www.itu.int/rec/T-REC-T.88-200306-S!Amd2/en) mid-2003.
+            // I haven't seen it being used in the wild, and the spec says "32-byte field as shown below" and then shows 24 bytes,
+            // so it's not clear how much data to read.
+            return Error::from_string_literal("JBIG2ImageDecoderPlugin: GBTEMPLATE=0 EXTTEMPLATE=1 not yet implemented");
+        }
+
+        size_t number_of_adaptive_template_pixels = arithmetic_coding_template == 0 ? 4 : 1;
+        if (data.size() < 2 * number_of_adaptive_template_pixels)
+            return Error::from_string_literal("JBIG2ImageDecoderPlugin: No adaptive template data");
+        for (size_t i = 0; i < number_of_adaptive_template_pixels; ++i) {
+            adaptive_template_pixels[i].x = static_cast<i8>(data[2 * i]);
+            adaptive_template_pixels[i].y = static_cast<i8>(data[2 * i + 1]);
         }
+        data = data.slice(2 * number_of_adaptive_template_pixels);
     }
 
     // 7.4.6.4 Decoding a generic region segment