[WebGL2.0][Firefox]Reference image seems wrong in Mul_dynamic_xxx tests

OS: w10_64bit
GPU: AMD/NV
browser: Firefox
config: enable webGL2.0, enable OpenGL rendering

Steps:
1)"https://www.khronos.org/registry/webgl/sdk/tests/webgl-conformance-tests.html?version=2.0.0"
2) run test "deqp/functional/gles3/shadermatrix/mul_dynamic_lowp.html"
3) test case "shader_matrix.mul.dynamic.lowp_mat2_mat2_vertex" failed, result is different with reference.

Attach the first 5 vertexes drawn in shader_matrix.mul.dynamic .lowp_mat2_mat2_vertex, according to my calculate, render result is ok.
[DYNAMIC_lowp_mat2xmat2.xlsx](https://github.com/KhronosGroup/WebGL/files/483968/DYNAMIC_lowp_mat2xmat2.xlsx)

PS: test failed in both AMD and NV asics.

Start by running the WebGL conformance test URL with the listed Firefox and WebGL 2.0 configuration. Reproduce deqp/functional/gles3/shadermatrix/mul_dynamic_lowp.html, specifically shader_matrix.mul.dynamic.lowp_mat2_mat2_vertex, and compare its reference result with the attached vertex calculations. Done means the source of the mismatch is identified and the affected conformance result or reference is corrected.

Written by the indexing model from the issue text.