Add support for WebGL2 MIN_PROGRAM_TEXEL_OFFSET

Improves the support of the WebGL2 `MIN_PROGRAM_TEXEL_OFFSET` property
(ie. stores it as a signed integer) and adds support for querying it
using GetParameter.

See: https://www.khronos.org/registry/webgl/specs/latest/2.0/#3.7.2

components/script/dom/webgl2renderingcontext.rs

@@ -995,6 +995,9 @@ impl WebGL2RenderingContextMethods for WebGL2RenderingContext {
             constants::MAX_UNIFORM_BLOCK_SIZE => {
                 return DoubleValue(self.base.limits().max_uniform_block_size as f64)
             },
+            constants::MIN_PROGRAM_TEXEL_OFFSET => {
+                return Int32Value(self.base.limits().min_program_texel_offset)
+            },
             _ => {},
         }