feat(core): support custom mime type for canvas loadImage

[DGCK81LNN]

Fix #51

[DGCK81LNN]

Actually this does not work for my use case because the loaded SVG image taints the canvas, making it throw an error when exporting. I had to use a data: URL in index.html instead of a Blob.

My SVG (generated code):

<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 320 160"><style>@font-face{font-family:"Iosevka Custom";font-stretch:semi-condensed;font-weight:bold;src:url("data:font/woff2;base64,[...]")}@font-face{font-family:"Sarasa Mono SC";src:url("data:font/woff2;base64,[...]")}@font-face{font-family:"Italian plates 1999";src:url("data:font/woff2;base64,[...]")}@font-face{font-family:"Code2000";src:url("data:font/woff2;base64,[...]")}@font-face{font-family:"方正姚体";src:url("data:font/woff2;base64,[...]")}.try{width:104px;height:100%;font-family:'Sarasa Mono SC';font-stretch:normal;font-weight:400;font-size:88px;line-height:160px}.plate:not(.plate-old) .try{position:absolute;left:0;top:0}.plate-small .try{width:65px;font-size:55px;line-height:100px}.try-wrap{display:inline}.try-wrap::after{content:" ";white-space:pre}.plate{position:relative;width:396.8px;height:160px;font-family:'Iosevka Custom';font-stretch:semi-condensed;font-weight:700;line-height:160px;font-size:112px;letter-spacing:-3.2px;padding-right:3.2px;text-align:center;font-optical-sizing:none}.plate-small{width:248px;height:100px;border-radius:5px;line-height:100px;font-size:70px;letter-spacing:-2px;padding-right:2px}.chair{position:absolute;left:0;top:0;z-index:100;width:100%;height:40px;font-family:'Code2000';font-stretch:normal;font-weight:normal;font-size:32px;line-height:40px;color:blue;letter-spacing:normal}.elec{position:absolute;left:0;top:0;z-index:100;width:100%;height:40px;font-family:'Sarasa Mono SC';font-weight:normal;font-size:24px;line-height:40px;letter-spacing:normal}.plate-small .elec{width:250px;height:25px;font-size:15px;line-height:25px}.plate-old{width:320px;height:160px;font-family:'Italian plates 1999';display:flex;font-weight:normal;padding:0;letter-spacing:normal}.plate-old.plate-small{width:200px;height:100px;padding:0}.plate-old .edge{width:8px}.plate-old.plate-small .edge{width:5px}.plate-old .middle{width:48px}.plate-old.plate-small .middle{width:30px}.plate-old .dot{width:16px}.plate-old.plate-small .dot{width:10px}.plate-old .try{width:96px;font-size:88px}.plate-old.plate-small .try{width:45px;font-family:"方正姚体";font-size:50px}.plate-old .middle5{width:56px}.plate-old .dot5{width:24px}.plate-old .edgelong{width:16px}.plate-old .middle4{width:64px}.plate-old .dot4{width:32px}.plate-old.plate-small .edgelong{width:10px}.plate-old.plate-small .middle4{width:40px}.plate-old.plate-small .dot4{width:20px}</style><path d="M8,0A8,8,0,0,0,0,8V152A8,8,0,0,0,8,160H312A8,8,0,0,0,320,152V8A8,8,0,0,0,312,0ZM55,25A5,5,0,0,1,55,15H75A5,5,0,0,1,75,25ZM245,25A5,5,0,0,1,245,15H265A5,5,0,0,1,265,25Z" fill="white"/><foreignObject x="0" y="0" width="320" height="160"><div xmlns="http://www.w3.org/1999/xhtml" class="plate plate-old" style="color: black"><div class="elec">电&#160;&#160;&#160;动</div><div class="chair">♿</div><div class="edge"> </div><div class="middle">A</div><div class="middle">B</div><div class="dot">·</div><div class="middle">1</div><div class="middle">2</div><div class="middle">3</div><div class="middle">4</div><div class="edge"> </div></div></foreignObject></svg>