Fix calls to glCcompressedTexImage2D with null pixel data

I believe this has always been an issue with emscripten's WebGL1 code
which only recently became more prevalent with emscripten-core#21445.

Fixes: emscripten-core#19300

Author: sbc100

src/library_webgl.js

@@ -1520,6 +1520,11 @@ for (/**@suppress{duplicate}*/var i = 0; i <= {{{ GL_POOL_TEMP_BUFFERS_SIZE }}};
   },
 
   glCompressedTexImage2D: (target, level, internalFormat, width, height, border, imageSize, data) => {
+    // `data` may be null here, which means "allocate uniniitalized space but
+    // don't upload" in GLES parlance, but `compressedTexImage2D` requires the
+    // final data parameter, so we simply pass a heap view starting at zero
+    // effectively uploading whatever happens to be near address zero.  See
+    // https://github.com/emscripten-core/emscripten/issues/19300.
 #if MAX_WEBGL_VERSION >= 2
     if ({{{ isCurrentContextWebGL2() }}}) {
       if (GLctx.currentPixelUnpackBufferBinding || !imageSize) {
@@ -1533,7 +1538,7 @@ for (/**@suppress{duplicate}*/var i = 0; i <= {{{ GL_POOL_TEMP_BUFFERS_SIZE }}};
     }
 #endif
 #if INCLUDE_WEBGL1_FALLBACK
-    GLctx.compressedTexImage2D(target, level, internalFormat, width, height, border, data ? {{{ makeHEAPView('U8', 'data', 'data+imageSize') }}} : null);
+    GLctx.compressedTexImage2D(target, level, internalFormat, width, height, border, {{{ makeHEAPView('U8', 'data', 'data+imageSize') }}});
 #endif
   },
 
@@ -1552,7 +1557,7 @@ for (/**@suppress{duplicate}*/var i = 0; i <= {{{ GL_POOL_TEMP_BUFFERS_SIZE }}};
     }
 #endif
 #if INCLUDE_WEBGL1_FALLBACK
-    GLctx.compressedTexSubImage2D(target, level, xoffset, yoffset, width, height, format, data ? {{{ makeHEAPView('U8', 'data', 'data+imageSize') }}} : null);
+    GLctx.compressedTexSubImage2D(target, level, xoffset, yoffset, width, height, format, {{{ makeHEAPView('U8', 'data', 'data+imageSize') }}});
 #endif
   },
 

test/browser/test_anisotropic.c

@@ -115,7 +115,12 @@ int main(int argc, char *argv[])
       while (level < 5) {
         printf("uploading level %d: %d, %d\n", level, w, h);
         assert(!glGetError());
+#if TEST_TEXSUBIMAGE
+        glCompressedTexImage2D(GL_TEXTURE_2D, level, GL_COMPRESSED_RGBA_S3TC_DXT5_EXT, w, h, 0, w*h, NULL);
+        glCompressedTexSubImage2D(GL_TEXTURE_2D, level, 0, 0, w, h, GL_COMPRESSED_RGBA_S3TC_DXT5_EXT, w*h, curr);
+#else
         glCompressedTexImage2D(GL_TEXTURE_2D, level, GL_COMPRESSED_RGBA_S3TC_DXT5_EXT, w, h, 0, w*h, curr);
+#endif
         assert(!glGetError());
         curr += MAX(w, 4)*MAX(h, 4);
         w /= 2;
@@ -136,7 +141,12 @@ int main(int argc, char *argv[])
       while (level < 5) {
         printf("uploading level %d: %d, %d\n", level, w, h);
         assert(!glGetError());
+#if TEST_TEXSUBIMAGE
+        glCompressedTexImage2D(GL_TEXTURE_2D, level, GL_COMPRESSED_RGBA_S3TC_DXT5_EXT, w, h, 0, w*h, NULL);
+        glCompressedTexSubImage2D(GL_TEXTURE_2D, level, 0, 0, w, h, GL_COMPRESSED_RGBA_S3TC_DXT5_EXT, w*h, curr);
+#else
         glCompressedTexImage2D(GL_TEXTURE_2D, level, GL_COMPRESSED_RGBA_S3TC_DXT5_EXT, w, h, 0, w*h, curr);
+#endif
         assert(!glGetError());
         curr += MAX(w, 4)*MAX(h, 4);
         w /= 2;

test/test_browser.py

@@ -2185,9 +2185,13 @@ def test_s3tc_ffp_only(self):
     self.reftest('s3tc.c', 's3tc.png', args=['--preload-file', 'screenshot.dds', '-sLEGACY_GL_EMULATION', '-sGL_FFP_ONLY', '-lGL', '-lSDL'])
 
   @requires_graphics_hardware
-  def test_anisotropic(self):
+  @parameterized({
+    '': ([],),
+    'subimage': (['-DTEST_TEXSUBIMAGE'],),
+  })
+  def test_anisotropic(self, args):
     shutil.copyfile(test_file('browser/water.dds'), 'water.dds')
-    self.reftest('test_anisotropic.c', 'test_anisotropic.png', reference_slack=2, args=['--preload-file', 'water.dds', '-sLEGACY_GL_EMULATION', '-lGL', '-lSDL', '-Wno-incompatible-pointer-types'])
+    self.reftest('test_anisotropic.c', 'test_anisotropic.png', reference_slack=2, args=['--preload-file', 'water.dds', '-sLEGACY_GL_EMULATION', '-lGL', '-lSDL', '-Wno-incompatible-pointer-types'] + args)
 
   @requires_graphics_hardware
   def test_tex_nonbyte(self):